What observation led researchers to purpose that chloroplasts evolved from cyanobacteria

Considering your talking about the a-d options Here is my answer
A) Both produce proteins.
B) Both contain mitochondria.
C) Both provide structure to cells.
D) Both perform photosynthesis.

Answer is D

Researchers were led to propose that chloroplasts evolved from cyanobacteria because they observed that both of them perform photosynthesis. 

answer: - b cells

explanation: -

there are many types of white blood cells present in our body. one such class of white blood cells are the lymphocytes. the b cells belong to the lymphocyte class of wbcs. the b cells perform their action against the pathogen by producing specialised proteins called as antibodies. these antibodies are produced as and when the b cells encounter pathogens in response to the antigens present on the surface of the pathogen. these antibodies bind to the antigens present on the pathogen and then, target them for destruction by other cells of the immune system.

so, the b cells mediate their action by identifying the antigens present on the surface of the pathogens and then producing antibodies in return to mediate its effect.

Lacking something of the ozone layer