What basis allows Carl and Carrie to claim Carol as a dependent?

Carl and Carrie can claim Carol as a dependent primarily based on her income status. For someone to be claimed as a dependent, they generally must not have earned income above a certain threshold if they are considered a qualifying child or a qualifying relative. Therefore, if Carol's income is below that limit, it supports the claim for Carl and Carrie to include her as a dependent.

The other factors mentioned—marital status and age—are also relevant but do not independently determine dependency. Marital status might affect whether she qualifies as a dependent if she is married, as a married individual filing jointly could disqualify her from being considered a dependent. Age can play a role in qualifying as a dependent for children, especially if they are under age 19 or a full-time student under age 24. However, since the question focused on what basis enables Carl and Carrie to claim her and the income status is a direct criterion for establishing dependency, that is the most accurate basis in this context.