Received a PM asking me to reply.

Core of the sentence is:

Tubman helped create what X and Y. (with X and Y parallel)

"helped" is simple past, so the sentence is in past tense.

X is "(had / would) become known as the UR"

Using "had" here would indicate past perfect tense. Past perfect is used when there are (at least) 2 past events, one of which happened before the other. The event that happened first / earliest would get the past perfect tense.

Here are the two actions:
Tubman helped create
what (had / would) become known as...

First, the UR has to be created. Then it can become known as the UR. So you can't use past perfect here for the later event.

Eliminate A and C.

D uses present perfect progressive: "has been becoming." The OG says that present perfect progressive "describes something that began in the past, continues into the present, and may continue into the future" (quote from OG11). Is that what's goign on here? No. Eliminate D.

That leaves us with either B or E, both of which use "would become." "Would" can be used to construct something called the "future in the past." This is when the verb's action lies in the future relative to the main verb in the same clause. In this case, we've got a main verb ("helped create") and an attached clause ("what would become known as..."). At the time that Tubman "helped create" this thing, it wasn't yet known as the UR. That was in the future, relative to that timeframe. So that's proper usage of a would / future in the past construction.