What form is the final verb in? Probably just mind blanking… I spend too much time on kanji and not enough on reading, writing and grammar …
折れる > 折れて (て form)
しまう > しまった (past tense)
のだ > んだ (not a verb, implies reason, figured I’d mention it anyway)
折れる is “to break”
しまう as an auxiliary verb means “to completely end up”, has a sense of irreversibility
They were probably asking about the んだ, at least that’s what it looked like.
But yeah, it’s not a verb form, it’s a shortening of のだ, which is used to state that it’s a reason for something. It’s sorta similar to “It’s (just) that…” in English.
And one other thing that might be confusing: The English translation given in the screenshot is written in the passive voice (the bat was broken by something), while the Japanese sentence is not. The key is that 折れる is an intransitive verb, meaning that the thing itself (the bat) does the action (breaks).
I think a translation that better captures the verb form used in Japanese would be, “My wooden bat broke because of that guy’s inside fastball.” Or if you want keep some of the nuances of んだ and しまった, then, “The reason my bat unfortunately broke is because of that guy’s inside fastball.”