Amachine: a- makes work easier to do

b- changes the size of the force needed

c- can change the direction or distance of the force

d- all above

A machine can do all the things that is stated above.

Option D

Explanation:

A machine is a device used to reduce the efforts put by human and to make our life easier so that we can spend our time and energy on some other useful things productively.

Machine reduces the force put by humans and alternatively plans and manages to equilaise the force required. It is also capable of changing the direction or the distance required by the force in a machine.

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.

electric field is a function of 1/r^2

yes, the force per unit charge for a point charge is inverse square law.