D, I chose.

Parallelism and past presence "was" is more appropriate than past perfect tense "had become."

To start, I think you need to understand proper comma usage. I love Grammar Girl's site for clear explanations, so here's what I'd read first:

https://www.quickanddirtytips.com/educat ... use-commas
https://www.quickanddirtytips.com/educat ... mma-splice
https://e-gmat.com/blogs/?p=575

Notice how proper comma usage is reliant on understanding how sentences are formed with independent and dependent clauses. Knowing how the GMAT uses dependent clauses (especially modifying clauses) is essential to rocking GMAT SC.

As to what to "look for" when searching for an error, whatever part of speech is underlined tells you what to check! For example, an underlined pronoun tells you to check for pronoun ambiguity or pronoun-antecedent agreement. An underlined participle tells you to check for a verb agreement or a modification error. If "much" or "many" is underlined, you check for a countable/uncountable diction error. This gets easier once you start categorizing every question you see (and every incorrect answer choice) by concept-tested.

This question actually has nothing to do with commas. It's tested parallelism and verb tense.

Deborah JOINED the army, WAS INJURED, and WAS DISCHARGED.

We have three past tense verbs that agree. "HAD BECOME" is correctly past perfect tense, since it is describing an event that took place even earlier back in the past. (She logically got ill BEFORE she was discharged.) The other choices break the parallelism, change the final verb tense to make the meaning illogical, or add transition words that convolute the meaning. Thus, (A) is correct.

Though you need to know what commas are and how they function, remember that Punctuation is NOT a tested GMAT concept in and of itself. Punctuation is merely a clue to help you identify GMAT errors such as Sentence Fragments, Run-ons, Parallelism, or Modification.