Q.Choose the correct option.
Acidic hydrogen is present in:
a. acetylene b. ethane c. ethylene d. dimethyl acetylene

Step 1. Recall which C-H is acidic. Section 15.3.4 states that only a TERMINAL alkyne's C-H (on an sp-hybridised carbon) is appreciably acidic, reacting with strong bases like lithium amide.

Step 2. Check each option. Acetylene (ethyne, H-C(triple bond)C-H) is a terminal alkyne with acidic H on both ends. Ethane (CH3-CH3) has only sp3 C-H, not acidic. Ethylene (CH2=CH2) has only sp2 C-H, not acidic. Dimethyl acetylene (but-2-yne, CH3-C(triple bond)C-CH3) IS an alkyne, but it is an INTERNAL one -- both triple-bond carbons carry a methyl group, not a hydrogen, so it has no acidic alkyne C-H at all.

Step 3. Conclude. Only acetylene among the four options has the specific terminal sp C-H that is acidic.

Checking each option for a terminal (sp-carbon) C-H, the only kind of hydrogen in this list that is meaningfully acidic

• Picking dimethyl acetylene just because it 'is an alkyne' -- being an alkyne is not sufficient; the acidic hydrogen specifically requires a TERMINAL triple bond, which but-2-yne lacks.
• Overlooking that ethylene's sp2 C-H, while somewhat more acidic than an alkane's, is still nowhere near as acidic as a terminal alkyne's sp C-H.