A pulsar pulses because a. its spin axis crosses Earth's line of sight. b. it spins. c. it has a strong magnetic field. d. its magnetic axis crosses Earth's line of sight.

A pulsar is a highly magnetized, rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. These beams are observed as pulses of radiation, including visible light, radio waves, X-rays, or gamma rays, that can be detected when the beam is pointed toward Earth. Pulsars were first discovered in 1967 by Jocelyn Bell Burnell and Antony Hewish, and they have since provided significant insights into the properties of neutron stars and the extreme physics in their vicinity.
Pulsars are incredibly dense and possess immense gravitational fields. Despite their small size—often just 20 kilometers in diameter—they can rotate several times per second. The intervals between pulses from a given pulsar are extremely regular, which makes them valuable for various applications, including tests of general relativity and even as cosmic clocks.
Understanding a pulsar's emission mechanism and why we detect these emissions as pulses requires diving into the intricacies of magnetic axes and their alignment with Earth's line of sight.

The magnetic axis of a pulsar is the line connecting its north and south magnetic poles. This axis is usually tilted with respect to the pulsar's rotational axis, which creates a dynamic scenario where the magnetic poles sweep through space as the pulsar rotates.
When the magnetic axis is at an angle to the rotation axis, it results in a beam of electromagnetic radiation emanating from the magnetic poles. It is essential to note that the emitted radiation from the magnetic axis is not constant in all directions. Instead, it is shaped like a lighthouse beam, shining out from the poles.
This misalignment between the rotation axis and magnetic axis is crucial for the pulsar to be observed from Earth. As the pulsar spins, the magnetic axis—and thus the beam of radiation—occasionally points toward Earth, making the pulsar appear to 'pulse' from our vantage point.

For the pulsation of a pulsar to be detected from Earth, the beam of electromagnetic radiation needs to intersect with Earth's line of sight. Imagine the Earth is watching a lighthouse. Every time the beam of light sweeps past, we see a flash, or pulse, of light.
In the case of a pulsar, the same principle applies. As the neutron star spins, if the magnetic axis crosses the line of sight from Earth, we observe a pulse. This alternating view of the beam causes the regular pulses associated with pulsars.
If the magnetic axis does not cross Earth's line of sight, the pulsar's emissions will remain undetected from our planet. Hence, not all pulsars are observed as pulsating objects; some may not align correctly with our line of sight and remain unseen.

A neutron star is the remnant core of a massive star that underwent a supernova explosion. These stars are incredibly dense; a single teaspoon of neutron-star material would weigh billions of tons. They are composed almost entirely of neutrons and have extremely powerful magnetic fields.
Neutron stars rotate very rapidly and can have rotational periods ranging from milliseconds to a few seconds. This rapid rotation makes them so fascinating, and when combined with strong magnetic fields, they create the conditions necessary for the pulsar phenomenon.
The study of neutron stars, including pulsars, provides scientists with a unique laboratory for understanding the behavior of matter under extreme conditions, that cannot be replicated on Earth. This research contributes to our broader comprehension of the universe, including aspects like gravitational waves and high-energy astrophysics.