Which characteristic do protozoans Foraminiferans (Phylum Sarcodines) and coccolithophores (Phylum Haptophyta) have in common?

The common characteristic of those two organisms is hard spherical shells (exoskeleton).

Foraminiferans are single cell marine eukaryotes divided into granular endoplasm and transparent ectoplasm. Foraminiferans are enveloped with tests, hard shells, usually composed of calcium carbonate (sometimes from organic compounds or silica).

Coccolithophore is a unicellular, eukaryotic alga with special calcium carbonate plates (or scales) of uncertain function (coccoliths). Each unicellular alga is enclosed in its own collection of coccoliths, the which make up its exoskeleton - coccosphere.

The other answer is correct, but more specifically:

D. They both develop calcium carbonate structures.

The addition of calcium carbonate makes exoskeletons harder/stronger.