A compound 'A' on reaction with 'X' and 'Y' produces the same major product but different by-products 'a' and 'b'. Oxidation of 'a' gives a substance produced by ants.

'X' and 'Y' respectively are

Both reactions involve cleavage of the terminal double bond in compound $$\mathrm{A}$$.

Cleavage produces the major ketone shown and a one-carbon by-product.

The by-product $$\mathrm{a}$$ on oxidation gives the substance produced by ants: $$\mathrm{HCOOH}$$.

Hence, by-product $$\mathrm{a}$$ must be: $$\mathrm{HCHO}$$ since oxidation of formaldehyde gives formic acid.

Therefore, reagent $$\mathrm{X}$$ must be a reagent capable of cleaving the terminal alkene to form formaldehyde.

Thus, the right option is D.