3) B

We know that the cyclist is moving forwards with acceleration 4 m/s2. This means that the cyclist needs to overcome the resistive forces in order to move forwards.

So we can conclude that 600 N > 300 N + air resistance. Hence, options D and E cannot be the right answers.

Resultant force is the sum of all the forces involved. (we must remember, that force is a vector quantity, hence the resistive forces would be negative, i.e., have a negative sign in equations).

Resultant force = 600 – (300 + air resistance) = 600 – 300 – air resistance

The resultant force would be the product of the acceleration produced by the cyclist and the combined mass of cyclist and bicycle. (The equation is Force = mass x acceleration)
Resultant force = 50 x 4 = 200 N

Using the equation, we created (Resultant force = 600 – 300 – air resistance), we get:
200 N = 600 – 300 – air resistance
If we rearrange, we get:
Air resistance = 600 – 300 – 200 = 100 N