Q:Which military alliance had Pakistan as its member?

The Central Treaty Organization (also referred to as CENTO, original name was Middle East Treaty Organization or METO, also known as the Baghdad Pact) was adopted in 1955 by Iran, Iraq, Pakistan, Turkey, and the United Kingdom. It was dissolved in 1979.

The Southeast Asia Treaty Organization (SEATO) was an international organization for collective defense in Southeast Asia created by the Southeast .SEATO was dissolved on 30 June 1977 after multiple members lost interest and withdrew.
If we talk about past then both Seato and Cento will be correct answer not only CENTO.