At a time when it was unusual to do it, Dorothy Sterling wrote about

Hi,

At a time when it was unusual to do it, Dorothy Sterling wrote about such major figures of Black history as Harriet Tubman and W. E. B. Du Bois.



I would first like to explain why “doing so” is preferable to “doing it”. Once you know the difference, you will have no problem in using pronouns correctly in such sentences.
Pronouns can only and only refer to “nouns” and “other pronouns”. It can refer to no other entity in the sentence. However, in the original sentence here, pronoun “it” has been used to refer the verb “wrote”. What was unusual at that time? To write about major figures of Black history was unusual. Therefore, here “it” is incorrectly referring to the “verb”. This makes the usage of “it” incorrect in this sentence. When we replace “it” with “so”, the entire phrase becomes “doing so” or “to do so”, a verb phrase that can easily refer to a verb, in this case “wrote”. This is reason why, “doing so” is not only preferred but is grammatically correct when it comes to refer to an action in the sentence.

Now let’s discuss choice C: doing that was unusual: This choice is incorrect because again relative pronoun “that” is referring to verb “wrote”. This is incorrect because this pronoun also can only refer to a noun.

Hope this helps.
Thanks.
Shraddha