Tense questions look like they are about grammar rules, but they are usually about context. The correct tense is whichever one matches the timeline the rest of the passage has already set, so the answer almost always comes from a verb in a nearby sentence rather than from a rule.
Start by finding the tense of the surrounding verbs. If the paragraph is describing a historical event and the verbs around the blank are past tense, the blank is past tense too. If the paragraph is describing what a theory holds or what a text does, the verbs will be present, and the blank follows. Consistency is the default expectation.
Departures from that default need a reason in the passage. A time marker such as in 1927, last summer, or currently can justify a shift. In: the museum now displays the collection, which curators assembled over two decades, the present and past tenses coexist because the sentence describes two different times, and the words now and over two decades mark them.
The past perfect, formed with had plus a past participle, is for an event that happened before another past event. In: by the time the team arrived, the equipment had already been unloaded, the unloading came first. If only one past event is being described, the past perfect is wrong, and the SAT offers it precisely to see whether you will pick the more complicated-sounding form.
Present perfect, formed with has or have plus a past participle, covers something that began in the past and continues or remains relevant. Researchers have studied the site for thirty years implies the study is ongoing. Researchers studied the site for thirty years implies it is finished. The choice between them depends on what the passage says about whether the situation continues.
Practically, never answer a tense question from the underlined sentence alone. Read the sentence before and the sentence after, note their verbs, and match unless the passage gives you an explicit reason to shift. The rule is the tiebreaker, and the context is the evidence.