Which verb is progressive?

a.
will think

b.
have thought

c.
thinks

d.
am thinking

Progressive verbs represent an action which was / is / will be happening. It always goes with dynamic verbs (unlike in static words, these are actions that may change somehow). In this case I think C - had been thinking - would be the progressive verb as it was happening in the past but we're not sure if the subject actually finished what he had been doing (he could change his mind and just stop thinking for example).