Step 1: Treaty facts.
Signed in 1960 by India and Pakistan with the World Bank as a signatory, allocating Indus system rivers (three eastern to India, three western to Pakistan).
Step 2: Eliminate others.
Bangladesh, China, and Nepal are not parties to the Indus basin arrangement governed by this treaty.
Step 3: Conclude.
\[
\boxed{\text{Pakistan (Option A).}}
\]