Step 1: Define the Pact’s role.
Signed in 1955, the Warsaw Pact created a unified military command and mutual-defense arrangement for the USSR and its satellite states.
Step 2: Principal objective.
Its central purpose was to balance and deter NATO by coordinating Eastern Bloc forces in Europe.
Step 3: Discard other options.
Keeping the USSR united (A) was a domestic political aim, not the pact’s treaty function; (B) is geographically off; (D) nuclear development was handled by the USSR, not by the Pact as an institution.
Step 4: Conclude.
\[
\boxed{\text{Countering NATO in Europe (Option C) was the core function.}}
\]