Apositron is emitted when *

an alpha particle forms
proton converts to a neutron
neutron converts to a proton

The correct answer is:
a positron is emitted when proton converts to a neutron.

The reaction can be described as following:
₁¹p (proton) → ₀¹n (neutron) + ₁°e (positron or ₁⁰β)
Positron is an antiparticle of a β particle (₋₁°β), which means it has an oposite charge to it, but same mass.