Refer to figure.
A wing stalls at or above a particular angle of attack, called the critical angle of attack. This is the angle between the relative airflow and the chord line of the wing. The angle of incidence, however, is the angle between the longitudinal axis of the aircraft (forward/backward axis) and the chord line of the wing. This is set by the designer and cannot be changed.

Normally, wings are designed so the incidence angle of the wing tip is smaller than the root (wing twist/washout). This means that, approaching the stall, the wing root will has the highest angle of attack and will therefore stall first.

Why should the wing root stall first?

• This preserves aileron control at higher angles of attack. The last thing you want to stall is your ailerons, as you want to be able to keep the aircraft's wings level, and they are mounted near the wing tips.
• This means the first sign of stalling wings is also coincident with the buffet of the tailplane, if the wing tips started to stall first then their turbulent wake would not make any buffet as it wouldn't hit the tailplane, but the wing root turbulence will.
• To avoid a violent wing drop. If the tip stalls first, then not only will you get no aileron use or possibly aileron reversal, but if one wing stalls even slightly before the other (which is pretty likely as perfect stalls are difficult to do), then the tendency to have a violent wing drop is much increased, causing a spin.

This all means that aircraft designers will always want the wing root to stall before the wing tip.

Therefore, the correct answer here is “prevent the wing tip from stalling.”.