The correct answer is a. Why is the pronoun it correct?

It was not long after the 1930s commenced that such baritone singers as Bing Crosby and Russ Columbo contributed to the popularization of a type of romantic, soothing singing that came to be called “crooning.”

a. It was not long after the 1930s commenced that such baritone singers as Bing Crosby and Russ Columbo contributed to

b. Not long after the commencement of the decade of the 1930s, baritone singers such as Bing Crosby and also Russ Columbo decided to contribute in

c. Not long after the 1930s commenced, baritone singers like Bing Crosby and Russ Columbo contributed to

d.Not long after the beginning of the 1930s commencement, baritone singers like Bing Crosby and Russ Columbo had contributed to

e.It was not long after the 1930s commenced that baritone singers such as Bing Crosby and Russ Columbo had contributed in

B -> 'contributed to' is correct idiom. Also, 'singers such as Bing Crosby and also Russ Columbo' is wrong. 'Also' should not be there.
C -> 'singers like Bing Crosby' is wrong. These are examples and the correct usage is 'such as'
D -> 'beginning of commencement' is awkward and redundant
E -> 'had contributed' implies the contribution happened before commencement of 1930s. That is not implied by the original sentence.

'It' in A does not need an antecedent. This is just 'part of the usage' for lack of better explanation on my part.

Thanks...but why doenst it need an antecedent?

Not sure brother. English not being my native language, I have a hard time with the SC rules. I go by my ear on things like this and it just 'sounds right'. Somebody from the staff (or somebody who knows) needs to explain.

Not every pronoun needs an antecedent in the same sentence. For example, "He is tall" is a perfectly correct sentence, even though "he" does not have an antecedent in the sentence.

In the case of "He is tall", however, we at least know that the pronoun is referring to someone (a male human being). In the case of "It was not long after the 1930s...", the "it" doesn't refer to anything at all. This may seem puzzling, but is conventional. We use "it" as a meaningless subject for expressions concerning time, weather, temperature, or distance. Thus we say:

It was raining hard.
It is hot outside.
It's a long way to Tipperary.
It has been years since I graduated from high school.

-Jad