Question

what is released through nuclear fusion in stars?
energy
gas
mass
pressure

Explanation:

Nuclear fusion in stars converts mass into energy according to Einstein's mass - energy equivalence principle ($E = mc^{2}$). It is energy that is released. Pressure is a force per unit area and not the output of nuclear fusion. Gas is the material involved in the process, not the released quantity. Mass is what gets converted, not what is released in the traditional sense.

Answer:

energy