What type of nuclear decay releases energy but not a particle?
a. alpha decay
b. beta decay
c. gamma decay
d. electron decay

Well,

A. I know that alpha decay is the process of expelling a helium atom, or two protons and two neutrons.  Those are particles.

B. Beta decay releases either a proton or an electron.  Those are also particles.

C. Gamma decay releases a gamma ray, or high-energy photon.  A photon is a particle.

D. I don't know much about electron decay, but since it is the only one left, I would go with this one by reason of deduction.