Question

because ct scanners can map objects that are impossible to excavate, ct scanning and 3-d printing can also be used to reproduce fossils that scientists cannot observe firsthand. for example, researchers from the national museum of brazil has relied on this technique to study a fossilized skeleton that was discovered protruding from a rock at an old são paulo railroad site
will rely
no change
is relying
relied

Explanation:

The subject "researchers" is plural, so the verb should be in the past tense to match the context of the action (studying the fossilized skeleton) that has already occurred. "Relied" is the simple past tense form that agrees with the plural subject and the context of a completed action. "Has relied" is singular (incorrect for plural "researchers"), "will rely" is future (doesn't fit the context of a past action), and "is relying" is present continuous (also doesn't fit the past context).

Answer:

D. relied