Fossils of thumb-length
primates that they had recovered from the fissure-filled sediments of a limestone quarry in China in 1996, easily represents the smallest known primates, with one species estimated to have weighed only 10 grams.
Option Elimination -
A. primates, that they had recovered from the fissure-filled sediments of a limestone quarry in China in 1996, easily represents the smallest known primates - no antecedent of "they." "that" acts as a relative pronoun modifying "primates," meaning "primates," which means "mammals" that had recovered from the sediments. We can't recover "live mammals" in the thousands of years old sediments, but we can recover their "fossils." Moreover, the subject is "fossils," which is plural, and the verb is singular, "represents." SV issues.
B. primates that were recovered from the fissure-filled sediments of a limestone quarry in China in 1996, are the smallest known primates - the same relative pronoun issue and wrong meaning. Moreover, "Fossils are the primates" is wrong. Fossils can't be mammals. Fossils can be of mammals.
C. primates, recovered from the fissure-filled sediments of a limestone quarry in China in 1996, easily represent the smallest known primates - ok
D. primates which were recovered from the fissure-filled sediments of a limestone quarry in China in 1996, easily represent the smallest known primates - "which" refers to "primates," creating the same meaning issue as in A
E. primates, recovered from the fissure-filled sediments of a limestone quarry in China in 1996, easily representing the smallest known primates - No verb.