Step 1: Key event.
In 1958, General Ayub Khan took power in Pakistan, initiating formal military rule.
Step 2: Compare cases.
Sri Lanka’s politics remained civilian-led; Nepal had monarchy transitions and later republicanism; Bhutan retained monarchic constitutional evolution—none saw earlier military takeover.
Step 3: Conclude.
\[
\boxed{\text{Pakistan (Option D).}}
\]