Final answer:
Pakistan became an independent republic in 1956, which is represented by the number b) 1950 on the timeline.
Step-by-step explanation:
The number on the timeline that represents the period in which Pakistan became an independent republic is b) 1950. Pakistan, initially composed of both East and West regions, gained independence in 1947 when Britain withdrew their control.
However, it wasn't until 1956 that Pakistan adopted its constitution and became an Islamic republic.