A leading-edge slat increases the wing’s maximum lift capability by shaping the airflow at high angles of attack. It creates a gap (slot) that lets high-energy air flow from below the wing to the upper surface, energizing the boundary layer and delaying flow separation. Because the airflow remains attached longer, the wing can reach a higher angle of attack before stalling, raising the stall angle. This effect is valuable during takeoff and landing when the wing operates near stall speed, and it isn’t limited to high speed or only when flaps are used. So the statement is true.

In an oblique shock, the airflow is decelerated and compressed, converting kinetic energy into internal energy. Since the flow is adiabatic, the stagnation (total) temperature stays the same across the shock. The normal component of the Mach number drops from before to after the shock, and because T0 = T [1 + (gamma-1)/2 M_n^2], a decrease in M_n behind the shock must be accompanied by an increase in static temperature T. Therefore, the static temperature behind the oblique shock is higher than ahead. This holds for any M1 > 1 with a calorically perfect gas like air, so it’s not indeterminate and not limited to certain Mach numbers.

Feathering a propeller means turning the blades to align with the oncoming air so the angle of attack is very small. With the blades oriented edge-on to the flow, the aerodynamic resistance is greatly reduced, so the propeller produces as little drag as possible while coasting or under engine failure conditions. This is why feathering is used to minimize the drag from the propeller and improve gliding performance. It’s not zero drag—there’s still some residual drag from the hub, bearings, and minor aerodynamic losses—but it is as small as practically achievable. So the statement is true because feathering reduces the propeller’s drag compared with other blade positions.

Lift is determined by the equation L = 1/2 ρ V^2 S CL, so the four factors that set its magnitude are the wing area (S), the air density (ρ), the velocity through the air (V), and the lift coefficient (CL). The lift coefficient is how effectively the wing converts inflow into lift, and in normal flight CL is governed mainly by angle of attack: increasing the angle strengthens the lift up to the point of stall. So increasing wing area adds more surface to generate lift, higher air density or faster speed increases the dynamic pressure acting on that surface, and a larger angle of attack raises CL and hence lift. Why the other options don’t fit: engine power doesn’t directly determine the instantaneous lift, and while altitude or temperature affect air density, they aren’t four direct determinants of lift themselves. Choices including surface roughness, color, humidity, or ground speed likewise don’t set the lift magnitude in the same direct way. The set that captures the essential factors is wing area, air density, velocity, and angle of attack (through CL).