Under what circumstance can a taxpayer not claim a dependent?

When a taxpayer claims the Married Filing Jointly status, they are combining their income and tax liability with their spouse. In this case, they cannot also claim another person as a dependent because the tax benefits associated with claiming a dependent are already included in the Married Filing Jointly status. This is why option D is the correct answer.

Options A and B are incorrect because a taxpayer can claim a dependent who is a US citizen or national, or a resident of the US, Canada, or Mexico, as long as the other requirements for claiming a dependent are met.

Option C is incorrect because a taxpayer can claim a dependent even if they are filing separately, as long as the dependency requirements are satisfied.