Which are correct about chain reactions?
A. Explosions can be expected when chain branching occurs.
B. A highly reactive intermediate reacts to produce another highly reactive intermediate.
C. In most of the radical chain reactions, the chain carrier is a radical.
D. The chain reaction usually involves initiation, chain propagation and chain termination steps

Hint: Before proceeding to the question let us first know basics about chain reactions. A chain reaction is a process in which neutrons generated during fission cause another fission in at least one additional nucleus. This nucleus, in turn, creates neutrons, and the cycle continues.

Complete answer:
A chain reaction is a process in which neutrons generated during fission cause another fission in at least one additional nucleus. This nucleus, in turn, creates neutrons, and the cycle continues. The process can be controlled (nuclear power) or uncontrolled (nuclear power) (nuclear weapons). The number of fissions doubles with each generation if each neutron produces two more neutrons.

Now, coming to the question, all of these are facts about chain reactions. A chain reaction is a succession of reactions that are started by another reaction. An unstable product of the first reaction is employed as a reactant in the second reaction, and so on until the system achieves stability. It often follows a pattern of initiation (the initial spark), propagation (the continuance of the process), and termination. Nuclear chain reactions are the most well-known kind of chain reactions.

Hence, the correct options are (A), (B), (C) and (D).

Note: It should be noted that positive feedback in a chain reaction leads to a self-amplifying chain of occurrences. Chain reactions are one method for systems that are not in thermodynamic equilibrium to release energy or raise entropy in order to achieve a higher entropy state.