Question

The doctor told us we had to leave immediately, which word in this sentence is an adverb

Answer 1

Immediately is the adverb

Answer 2

Adverb- A part of speech that modifies a verb, an adjective, or another adverb. Adverbs usually answer questions as, "How?, Where?, When?, or To What Degree?".

The adverb is immediately. The adverb refers to time and describes the verb leave. 

Hope this helps you! =)