What starting material is needed to prepare each compound by a ring-closing metathesis reaction?

In alkenes metathesis, also known as olefins metathesis, two alkenes molecules react with each other, and the carbon atoms of each C=C are inter-changed.

Hence, in this reaction, two σ and two π bonds are broken, and two σ and two π bonds are made.

The reaction in a very dilute solution favors the ring-closing metathesis due to the higher possibility of the two alkene ends interacting with each other rather than with the functional groups in other molecules or compounds.

The double bonds in the given products are cleaved, and each carbon atom in the C=C is bonded with a $={\mathrm{CH}}_2$.

Starting material of a

Starting material of b

c. The double-bond in the given reaction is broken, and two double bonds are formed at the terminal positions. The starting material for the given product is given below.

Starting material of c