Concept:Algeria's most important oil fields are in the Sahara Desert, with Hassi Messaoud being the largest.Explanation:Hassi Messaoud is a major oil field in southeastern Algeria.Touggourt is an Algerian oasis town, not an oil field.Marsa el Brega and Gulf of Sirte are in Libya, not Algeria.Therefore, the correct location is Hassi Messaoud.Answer:A. Hassi Messaoud