First Principles Thinking: effective tax rate = income tax expense ÷ pre-tax income
B is correct. The CFA Curriculum defines the effective tax rate as the reported income tax expense divided by pre-tax income. Here, USD 30,000 ÷ USD 200,000 = 15%. This rate may differ from the statutory rate due to permanent differences, tax credits, and cross-border income.
A (10%) would imply an income tax expense of USD 20,000, which is not consistent with the given data.
C (20%) would imply an income tax expense of USD 40,000, also inconsistent with the data.