Which statement about forces is true?
forces are defined by strength but not direction.
forces are defined by direction but not strength.
forces are defined by both strength and direction.
forces act only on objects that are in contact.

The answer would be:

Forces are defined by both strength and direction.

Here is why:

Force is what you call a VECTOR quantity. It is a quantity that is described by MAGNITUDE and DIRECTION. The strength of a force is its magnitude and the direction, well it is the direction the magnitude is applied on the object.

You cannot fully describe forces without both these quantities. Think of it this way, if two people are pushing a car, how would you know if the car is moving if you do not know in what direction are the forces applied and how much strength was used.

It is almost like asking a blind man to catch a ball without even calling out to them before throwing it. He would not know where to face, or how strong you will throw it.