Oh, I do hope this question isn't from a quiz. If so, it is poorly formed and ambiguous given the sentence you have to work with, and you have my sympathy.
But let's have a go at it. We'll assume "main subject" means the subject of the main clause, which is "they became interested in determining the behaviors associated with each status". (The rest of it, "Once researchers were able to identify rejected and neglected children ..." is a dependent clause that modifies the rest, but you should be able to ignore this for the purpose of this analysis.) So the pronoun "they" is the subject of the main clause, and so what may be called the main subject (although I have a sneaking suspicion they are after "researchers" instead).
Similarly, the "main verb" can be identified as "became".
Now, the bit that gives it away as a badly-formed question -- there is no object for that verb. "Became" is a linking verb, and it has a subject complement instead of an object. That complement here is the adjectival phrase "interested in determining the behaviors associated with each status".