The option that can be used as a one-word substitute for the given group of words is "Relapse."

Explanation of the options:

☀ Rekindle: This word means to ignite or revive something that was previously extinguished or weakened.

☀ Reiterate: This word means to repeat or emphasize something that has already been said or done.

☀ Relayed: This word means to transmit or pass on information or a message to someone else.

☀ Relapse: This word specifically refers to falling back or sinking again into a worse state after experiencing an improvement or recovery. It is commonly used in the context of health or addiction-related issues.