"Did a bird just chirp?"
If you come here asking why it uses が not は: you are introducing a new subject.
Why are we using 今 here? Isn't 今 now? This sentence is in the past tense, right? Can anyone please clarify my confusion?
Because we end with ました. Indicating past tense. So 今 becomes "Now of the past". Which Duo describes as "just now".
Using ばかり or ところ here doesn't sound correct because it puts the emphasis on the time since the chirping; therefore it makes it sound like you're asking if it was just now that the bird chirped.
You could maybe use たった今 to put an emphasis on the "just", but it's really not necessary.
uh, yeah, which is why it's the correct answer. Perhaps you're not understanding the English sentence?