Ateam of researchers reported that five endangered birds species had resumed nesting or investigated nesting boxes erected by volunteers having been ferried out to the island last month.
a. investigated nesting boxes erected by volunteers have been
b. begun investigating nesting boxes erected by volunteers, being
c. investigating nesting boxes, erected by volunteers,
d. were investigating nesting boxes erected by volunteers
e. investigate nesting boxes erected by volunteers, which

C). Investigating nesting boxes, erected by volunteers.

Explanation:

As per the question, option C best revises the underlined phrase as it proposes a grammatically correct and parallel structure to the sentence. As per the rules, coordinating conjunctions like 'or' join two words or phrases carrying equal significance and according to which all the options(A, B, D, and E) go incorrect except the option C. It follows the 'ing' form of the previous noun 'nesting' and employs 'investing' so that both becomes grammatically equal and also employs appropriate punctuation. Thus, option C is the correct answer.

c. explains words’ origins and variations.

answer: do

the 'italicized phrase' is a noun clause that is used as a direct object.