In math classes in the US fifty years ago, this was called A prime.
It depends on where you come from. It could be A (as in uh) or A (as in eh). Both would be correct.
A prime. If you said "A line," I would probably think of "A bar." "A bar" has a solid horizontal line above it. I tried to copy and paste it here, but it didn't work.
By the way,
A''= A double prime
A'''= A triple prime