Photosynthetic protists with shells, composed of two halves, that fit together like a petri dish are:

Explanation

The correct answer is diatoms. Diatoms are a group of photosynthetic protists known for their unique silica shells, which are composed of two halves that fit together like a petri dish. This distinguishing feature sets them apart from other protists. Actinopods have shells with a different structure and are not primarily photosynthetic. Foraminiferans possess calcareous shells and are not typically photosynthetic. Dinoflagellates have cellulose plates, not shells that fit together in the manner described in the question.