Consider the depicted hydrogen (H) in the hydrocarbons given below. The most acidic hydrogen (H) is

The acidity of a particular hydrogen atom is judged by the stability of the conjugate base obtained after removing that hydrogen as a proton, $$H^+$$. The more stable the conjugate base (carbanion), the stronger (more acidic) the original C-H bond.

Four main factors control the stability of a carbanion:
  1. Hybridisation of the carbon bearing the negative charge: $$sp \; (50\%\,s) \gt sp^2 \; (33\%\,s) \gt sp^3 \; (25\%\,s)$$.
  2. Resonance delocalisation of the negative charge.
  3. Inductive $$(-I)$$ withdrawal by electronegative or electron-withdrawing groups.
  4. Destabilising $$+I$$ effects of alkyl groups (they increase electron density on the already negative carbon).

Analyse each option by applying these points:

Case A: The depicted hydrogen is attached to a simple primary $$sp^3$$ carbon of an alkane. Removing it gives a primary carbanion that is not resonance-stabilised and is slightly destabilised by the adjacent alkyl group’s $$+I$$ effect. Hence, very weakly acidic.

Case B: The depicted hydrogen is the terminal proton of an alkyne (≡C-H). That carbon is $$sp$$ hybridised, so it possesses 50 % $$s$$-character. Higher $$s$$-character means the electron pair in the conjugate base is held closer to the nucleus, making the carbon more electronegative and the negative charge more stabilised. No other option offers a carbanion on an $$sp$$ carbon; therefore this hydrogen is expected to be the most acidic.

Case C: The hydrogen is allylic/benzylic. After deprotonation the negative charge resides on an $$sp^2$$ carbon and is resonance-delocalised, giving moderate acidity. However, the $$sp^2$$ carbon (33 % $$s$$) is still less electronegative than an $$sp$$ carbon, so it is less stabilised than the acetylide anion from Case B.

Case D: The hydrogen is attached to a tertiary $$sp^3$$ carbon. The resulting tertiary carbanion is the least stable because three alkyl groups donate electron density by the $$+I$$ effect. Consequently, this hydrogen is the least acidic of all four.

Arranging in increasing order of acidity (weakest acid first):
Case D < Case A < Case C < Case B

Thus, the terminal alkyne hydrogen in Option B is the most acidic.

Option B which is: the terminal alkyne (≡C-H) hydrogen.