Step 1: Place each alliance in the Cold War camps.
NATO (1949), SEATO (1954) and CENTO (1955) were U.S.–led or Western-sponsored security arrangements.
The Warsaw Pact (1955) was created by the USSR and its Eastern European allies.
Step 2: Purpose contrast.
Western alliances aimed to contain Soviet influence; the Warsaw Pact institutionalized a Soviet-led military command to balance NATO.
Step 3: Conclude.
\[
\boxed{\text{Warsaw Pact (Option D) was an {Eastern Bloc} alliance, not Western.}}
\]