Increasing the wing lift coefficient gradient is not a way to make an aircraft (more) longitudinally statically stable. The lift coefficient gradient refers to the rate at which lift increases with angle of attack. A steeper lift coefficient gradient leads to a more abrupt change in lift as the angle of attack changes, making the aircraft less stable. Longitudinal stability is typically achieved by carefully designing the distribution of lift and center of gravity, and not by manipulating the lift coefficient gradient alone.
Decreasing the size or moving the position of the horizontal stabilizer is not a way to make an aircraft (more) longitudinally statically stable.
Decreasing the wing surface area
Decreasing the wing surface area would actually make an aircraft less longitudinally statically stable. Increasing the wing surface area, on the other hand, can improve longitudinal stability by providing more lift and stability to counteract pitch variations.
Increasing the horizontal tail surface area
Increasing the horizontal tail surface area is a way to make an aircraft (more) longitudinally statically stable. This increases the moment arm of the horizontal tail, allowing it to produce more stabilizing force and counteract any pitching moments caused by changes in angle of attack. By increasing the horizontal tail surface area, the aircraft becomes more resistant to pitch changes and can maintain a more stable longitudinal trim.
