why is alternating p.d needed in a cyclotron?

It's needed to create a magnetic field, confining the particle to the circular path.
An AC current changes the magnetic flux, which by definition gives rise to a magnetic field. A DC current won't have this effect

Have you had a read about what a cyclotron is before attempting the questions?
e.g. http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/cyclot.html

DC would only be able to accelerate charged particles in one direction - AC is able to accelerate particles in one direction in one half cycle and the opposite direction in the other half cycle.

thank you!!

Ignore what I said - totally wrong. Been a long day haha
Sorry