In which type of reaction do two lighter nuclei combine to form one heavier nucleus?
(1) combustion (3) nuclear fission
(2) reduction (4) nuclear fusion

Nuclear Fusion

Explanation:

In nuclear fusion two lighter nuclei combine together in order to form one heavier nucleus.

For example, the energy generated from the Sun is made by nuclear fusion: Two hydrogen (H, atomic number 1) nuclei combine in order to form one helium  (He, atomic number 2) nucleus.

Nuclear fission is the opposite reaction, in which one heavier nucleus splits into two lighter nuclei.

the answer to this question is 3.