...... was the only significant imperial power in the Northern half of Africa at the end of the 18th century

A.

Ottoman Turkey

B.

Italo-Egyptian Empire

C.

Ottoman-Egyptian

D.

Portuguese Empire

Correct answer is A

Ottoman Turkey is the correct answer. Its dependencies in Egypt, Tripoli, Tunis and Algiers were admittedly almost self-governing, but they did at least contribute some revenue to the sultans in Istanbul.