Identify the visible option

The only clearly visible compound in the prompt is:

$\text{A: } \mathrm{CH_3-C\equiv C-H}$

This is a terminal alkyne (propyne), which contains an acidic hydrogen attached to an $sp$-hybridized carbon.

Recall the factor governing acidity

In organic compounds, acidity of a hydrogen depends on the stability of the conjugate base formed after removal of $H^+$.

For a terminal alkyne:

$\mathrm{CH_3-C\equiv C-H \rightarrow CH_3-C\equiv C^- + H^+}$

The negative charge is on an $sp$ carbon, which has 50% s-character. Higher $s$-character means the electron pair is held closer to the nucleus, making the anion relatively more stable and the hydrogen more acidic.

Typical acidity order

Among common hydrocarbons:

$sp\text{-C-H} > sp^2\text{-C-H} > sp^3\text{-C-H}$

Numerically, terminal alkynes have $pK_a \approx 25$, which is significantly more acidic than alkenes or alkanes.

Issue with the given options

However, options B, C, and D are not shown in the question image/text. Therefore, a full comparison among all four compounds is impossible from the provided information.

Conclusion from available data

Based only on the visible option, option A certainly has an acidic hydrogen and is more acidic than typical alkane/alkene C–H hydrogens. Since the other options are missing, I cannot justify selecting D.

Comparison with stored answer

The stored correct answer is D, but with incomplete option data, this cannot be verified. From the information shown, the best supported choice is A.