Any of them can be used, though they come about in different ways.
C. is the canonic Pseudo-Cleft construction, with an infinitive VP wash the sink as the complement of the Active pro-verb do required by pseudo-clefting. Note that one can optionally insert (or delete, if one goes to a different syntactic church) the infinitive complementizer to in front of wash.
B. is a variant pseudo-cleft, a reduction of what you have done is (you have) washed the sink, with the past participle washed being required after the perfective have.
A. is perhaps the least likely. In it, the gerund phrase washing the sink (you can tell it's really a gerund and not just a noun because it has a direct object) is the complement of what you have done. Note that they can be swapped, salva veritate:
- Washing the sink is what you have done.
English has thousands of such constructions, and each has its own set of affordances, prohibitions, requirements, and idioms. In many cases they are effectively synonymous; in others they can contrast. This is why we have such a variety; most of them are still in beta.