Which factor could prevent a person from being claimed as a dependent?

A person could be prevented from being claimed as a dependent primarily based on their income level. The Internal Revenue Service (IRS) sets specific income thresholds that, if exceeded, disqualify an individual from being a qualifying child or qualifying relative for dependency purposes. For example, if a person earns more than the prescribed limit, they are no longer eligible to be claimed as a dependent, regardless of their other attributes such as age, citizenship, or residency.

Individuals over the age of 18 can still be claimed as dependents under certain conditions, particularly if they are full-time students or meet other requirements. Being a U.S. citizen does not affect dependency status, as non-citizens can also be dependents if they meet certain criteria. Additionally, residency in Canada or Mexico can affect dependency claims, but this is typically more relevant for determining foreign tax credits or exclusions, rather than determining eligibility to be claimed as a dependent on U.S. tax returns. Thus, the income level is a decisive factor in determining dependency status, making it the correct answer.