Answer:
A. Reduces the effort force
Explanation:
In a general form, an effort force is a force applied on a point of a machine to do work (move something, for example) and overcome a resistance force. In this context, the effort force is the one which the rider applies with his feet on the pedal, to rotate subsequently: the crank, the chain wheel, the chain, and the wheels. The resistance force is the force caused by the friction with the air. Check the file I attach with the parts of a bicycle.
The force applied on the pedal creates a torque, which is transmitted (through the crank) to the chain wheel for it to rotate. Without that torque, the chain wheel couldn't rotate.
The reason why the bicycle uses a pedal and a crank is because they ease the application of that torque. Thanks to the length of the crank, is possible to obtain great amounts of torque (capable of moving a bicycle with a person on it) with the application of small effort force in the pedals.
Now, let's examine the four possible correct answers.
A. ✓ reduces the effort force: Yes, as stated in the last paragraph of the explanation, the pedal and the crank make possible to apply only a small amount of force on that pedal.
B. X increases the effort force: No, this would contradict to the said in the previous paragraph.
C. X reduces the resistance force: for the same velocity of the rider, the resistance force is the same and doesn't depend directly on the pedals.
D. increases the resistance force: again, the resistance force doesn't change for the same velocity
