Step 1: Cause–effect linkage.
NATO’s consolidation prompted the Soviet bloc to create the Warsaw Pact (1955) as a counterweight.
Step 2: Eliminate unrelated bodies.
SEATO is Asia–Pacific and U.S.–led but not the specific trigger; OPEC and the UN are not military alliances.
Step 3: Conclude.
\[
\boxed{\text{Warsaw Pact countered NATO (Option B).}}
\]