Answer: a. Concealment
Explanation: Nick has committed concealment. A concealment is the act of hiding or not putting forward any relevant information that need to be revealed to an isurer. This usually could affect the terms of the policy. This also includes situations in which an insurance company does not put forward question about the information directly. Intentionally or unintentionally the act may lead to loss to the insurer.
Insurance policies generally put together concealment and misrepresentation (a false statement of a material fact made by one party which affects the other party's decision in agreeing to a contract) in order not to void or alter a contract.