What benefit does sweepback provide to an aircraft?

Sweepback provides significant aerodynamic advantages to an aircraft, primarily by increasing the critical Mach number. The critical Mach number is the speed at which airflow over any part of the aircraft reaches the speed of sound. When an aircraft has sweepback wings, the effective angle at which the airflow meets the wing is reduced, allowing the aircraft to operate at higher speeds before encountering shock waves and the associated adverse effects of transonic and supersonic flight.

As the airfoil is swept back, the airflow over the wing tends to move in a way that postpones the onset of compressibility effects that usually occur at lower speeds on straight wings. By increasing the critical Mach number, aircraft with sweepback wings can achieve greater speeds and improved performance without experiencing detrimental aerodynamic effects, enabling more fuel-efficient high-speed flight.

The other options do not accurately reflect the advantages that sweepback wings provide. For instance, increasing drag is typically not a desired outcome, and while lift characteristics can vary with design, sweepback does not inherently decrease lift overall. Additionally, fuel efficiency is generally improved due to enhanced aerodynamic performance at higher speeds, not decreased.