Operating margin

Operating income is $210 thousand ($900 − $540 − $150). Divide by the same-period $900 thousand of revenue: 23.3%.

This includes the warehouse gain in operating income. The gain is below that subtotal.

This is gross margin, which excludes operating expenses.

This deducts the $30 thousand interest expense before calculating operating margin, but interest is below operating income.

Answer: a

Operating margin is 23.3%: $210 thousand of operating income ($900 − $540 − $150) divided by $900 thousand of revenue.