Step 1: Diplomatic fact.
India issued formal recognition to Bangladesh on 6 December 1971, acknowledging the new state's government during the Liberation War.
Step 2: Contextualize the other dates.
3 Dec—war began; 16 Dec—Pakistani forces surrendered (Victory Day); 26 Mar—Bangladesh’s independence declaration.
Step 3: Conclude.
\[
\boxed{\text{6 December 1971 (Option B).}}
\]