Answer: Only the effective tax rate.

Under US GAAP, the recognition of a valuation allowance for deferred tax assets increases the effective tax rate when recognized, as it raises income tax expense. This occurs because establishing a valuation allowance reduces the deferred tax asset and income in the period it is recognized. The effective tax rate is calculated as income tax expense divided by pretax income (accounting profit). Therefore, the correct answer is **A**, as it solely impacts the effective tax rate.

Author: LeetQuiz Editorial Team