Which statement about forces is true?

a. forces are defined by strength but not direction.
b. forces are defined by direction but not strength.
c. forces are defined by both strength and direction.
d. forces act only on objects that are in contact.

Let's look at each answer. A is incorrect because F=ma which is a vector. B is incorrect because F=ma which has both direction and magnitude. C is correct because force has both direction and magnitude. D is incorrect. Think for example of gravity which applies a force without direct contact.