(b) (B) as ketones are less reactive towards addition of HCN than aldehydes and alkane due to higher hinderance caused by steric effect and inductive effect.

(c) The possible non-cyclic functional isomers of a carbonyl compound having molecular formula.

‌C4H8O‌ are ‌−
‌

CH3−CH2

(I)

−CH2−CHO
CH3−

O

∥ C

(II)

−CH2−CH3

CH− CH | CH3 −CHO

(III)

Since isomer (B) does not give Tollen's test, it must be a ketone but it gives positive iodofo on test, so it must be methyl ketone. Hence, structure of (B) is (II).
The isomers (A) and (C) give positive Tollen's test so both the isomers are aldehyder. Since isomers (A) and (B) on reduction with Zn∕Hg / conc. HCl give the some product (D).

CH3− O ∥ C −CH2−CH3

(B)

‌

Zn(Hg)

────▸

‌ conc. ‌HCl

CH3−CH2−CH2−CH3

(D)

∴ Structure of (A) is (I) and Structure of (C) is (III).
Hence,
A‌⇒CH3−CH2−CH2−CHO
B‌⇒CH3−C−

O

∥ O

−CH2−CH3
C‌⇒CH3−

CH |

CH3

−CHO
D‌⇒CH3−CH2−CH2−CH3