Which of the following is true of chloroplasts?
a. they're composed of a liquid-filled space known as the thylakoids.
b. they're found in animal cells.
c. they're the main site of photosynthesis in autotrophs.
d. they're full of flattened stacks of membranes called stroma.

The best answer is C.

The word chloroplast is derived from the Greek word "chloros"  meaning "green" ,  and  "plastes"  meaning  "the one who forms"

Chloroplasts are cellular organelles found in autotrophs namely  green plants and are filled with the green pigment called chlorophyll.

These organelles conduct photosynthesis. With the help of chlorophyll, they absorb sunlight and convert it into sugar molecules and also produce free energy stored in the form of ATP and NADPH through photosynthesis.  

The leaves on the  plants have the highest concentration of chloroplasts.