A brief explanation can be found in the Tips and Notes of the Verbal Noun skill; my understanding is that the Tips and Notes aren’t available in the apps. Additional details beyond what the Tips and Notes contain can be found at the Syntax of the Verbal Noun page at Gramadach na Gaeilge.
Tá siad á ndúnadh - "They are closing them" (active voice, siad indicates who is doing the closing, ndúnadh indicates that plural things are being acted on, ie "closing them")
Tá siad á dhúnadh - "They are closing it/him" ("it" referring to a masculine noun)
Tá siad á dúnadh - "They are closing it/her" ("it" referring to a feminine noun)
Tá siad á ndúnadh agam - "They are being closed by me" (passive voice, siad indicate that "they" are being acted on, ndúnadh agreeing with siad)
Tá sé á dhúnadh agam - "it/he is being closed by me" Tá sí á dúnadh agam - "it/she is being closed by me"
The presence of an ag clause specifying an agent causes the sentence to be interpreted in the passive voice, which causes a change in the conjugation of the English verb.
It's not a test - it doesn't matter if you get it wrong, as long as you learn from that mistake.
But having said that, the siad tells you that you are dealing with a plural, so á ndúnadh is the correct form. (Tá sé á dhúnadh agam would be "he/it is being closed by me" and ta sí á dúnadh agam would be "she/it is being closed by me").
This had me totally confused. It isn't bad enough that I can never get these sentences that use "á". For some reason the "ag" sentences are easy for me and make sense but I am never sure what the "á" ones are supposed to mean but this one had "They are" plus "agam" which I had only known as meaning "I have", apparently it can mean "by me" as well. Interesting. Now if I can just find one of these that has audio so I will know how to say ndúnadh so maybe I could remember it as a form of 'close"...