Which of the following is true about inactivated (killed) vaccines?

Correct option is A

Killed vaccines are safe but weakly immunogenic, inducing mainly humoral immunity. Hence, multiple booster doses are needed to sustain protection.
Explanation of options: (a) ✔ Correct; killed vaccines (e.g., polio, hepatitis A) require boosters. (b) ✘ Only live vaccines can revert to virulence, not killed ones. (c) ✘ Cellular immunity is weaker compared to live vaccines. (d) ✘ Inactivated vaccines are safe in pregnancy (e.g., influenza).