Rank the compounds in each of the following groups from strongest acid to weakest acid

The acidic character depends upon the % S character in the molecule.
Higher is the S character,Higher is the acidity. The order of acidic strength in hybridization is

sp > sp²> sp³

a)

The ethyne molecule is most acidic compared to others due to more s character whereas in the case of acetaldehyde and ethene which have similar hybridization. “O” atom being more electronegative (in the case of acetaldehyde).Although a +I group is attached to the carbonyl carbon, here resonance will dominate for deciding the acidic strength the alpha H will be more acidic to form enolate ion. Thus, acetaldehyde is more acidic than ethene. Followed by ethane.

The order is represented as follows:

b)

In this case, the two carbonyl carbon attached to two OCH₃ group are more acidic as there will be resonance between the two oxygen, followed by two carbonyl carbon with one OCH₃ group, and then two carbonyl groups with H and lastly formaldehyde as it doesn’t contain $\alpha$hydrogen.

The order is represented as follows:

c)

The oxygen atom is more electronegative and is more acidic than the N atom followed by the nitrogen atom containing $\alpha$hydrogens is more acidic, and lastly, cyclohexanone which doesn’t involve resonance nor it has $\alpha$hydrogen.

The order is represented as follows: