Question

what observation led researchers to propose that chloroplasts evolved from cyanobacteria?
both produce proteins.
both contain mitochondria.
both provide structure to cells.
both perform photosynthesis.

Explanation:

Cyanobacteria are photosynthetic prokaryotes. Chloroplasts are the organelles in plant cells that carry out photosynthesis. The key similarity that led to the endosymbiotic theory (proposing chloroplasts evolved from cyanobacteria) is their shared ability to perform photosynthesis. Producing proteins is a general cellular function (not unique to this relationship). Chloroplasts and cyanobacteria do not contain mitochondria. Providing structure to cells is not a function associated with either's key similarity relevant to this evolutionary proposal.

Answer:

Both perform photosynthesis.