Which of the following structures is most likely to be found in an autotrophic protist?

b. A chloroplast 

The options in the question contain both autotrophs and heterotrophs.
(a) A giant redwood tree = Autotroph
(b) A spider = Heterotroph
(c) A rose bush = Autotroph
(d) A mushroom = Heterotroph
(e) A blue whale = Heterotroph
Autrophs are the organisms that have the capacity to form their own food from inorganic substances like the carbon dioxide. 
Heterotrophs are the organisms that cannot produce their own food but depends on other animals or plants for their nutritional needs.