D is incorrect because it uses past perfect instead of past. Past perfect tells us that an event occurred
before an event in the past tense.
In this sentence, Lewis and Clark
were (past tense) not the first white men to cross the continent. If we then say they
had not
visited (past perfect tense) places unseen/unmapped by native people, we're establishing a timeline — the past perfect event happened
before the past event. So they visited these places that they weren't the first to see
before they crossed the continent that they weren't the first to cross? If we're familiar with American history, we know that Lewis and Clark visited places
as they crossed the continent. So both verbs should be in the same tense.
If we aren't so familiar with American history, we can look at parallelism: we're looking at two things Lewis and Clark did but weren't the first to do. In most cases, if two verbs have the same subject and are parallel to each other (any of our FANBOYS, including "nor", are classic ways to set up parallelism), they will have the same tense. So because we have past tense in the first part ("were") we want past tense in the second part ("did").
General rule: don't use past perfect unless you need to. Past perfect means something very specific, so if you aren't looking to create a timeline where one event occurs before another past event, stick to simple past.