A property’s cost basis is most affected by an owner’s:

Debt service (principal and interest payments) are financing costs that don't affect basis. They're either deductible interest expenses or reductions of loan principal, neither of which changes the property's original cost basis.

Interest expense is tax-deductible but doesn't increase basis. It's an operational cost that reduces taxable income each year but doesn't modify the property's original cost basis or value.

Option D is incorrect because costs of improvements actually increase the property's cost basis, not affect it in a negative way. Capital improvements enhance the property's value or prolong its useful life, and these costs are added to the original basis. The question asks what most affects the basis, while improvements increase it rather than being the primary factor that defines or changes the basis itself.