Question

A team 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 having 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

Answer 1

D) were investigating nesting boxes erected by volunteers

This one sounds the best, I may be wrong.