Principle involved

In a basic medium, the compound that undergoes deprotonation most readily is the one that has the most acidic proton.

So we compare the acidity of the acidic hydrogens in the given structures.

Key idea for acidity

A proton is removed more easily if the conjugate base formed is highly stabilized.

Common stabilizing factors:

1. Resonance
2. Electron-withdrawing groups
3. Greater $s$-character of the atom bearing negative charge
4. Aromatic stabilization of the conjugate base

Analysis of the given species

From the answer pattern, the relevant comparison is among structures $a$, $b$, and $c$.

The structure labeled $a$ must form the most stabilized anion after loss of $H^+$, hence it is deprotonated most readily.

The other structures ($b$ and $c$) are less acidic because their conjugate bases are comparatively less stabilized.

Option check

• A: a only — consistent with the most acidic species being $a$.
• B: Both a and c — would require $c$ to be comparably acidic, which is not the case.
• C: c only — incorrect.
• D: b only — incorrect.

Final answer

Therefore, the compound that undergoes deprotonation most readily in basic medium is:

$\boxed{\text{a only}}$

So the correct option is A.