Why can work be thought of as the transfer of energy?it's physics

That is because work requires energy. According to the law of conservation of energy, it cannot be created or destroyed. When doing work, energy change forms and gets transferred to the object until it is released.
for example, when you lift up an object and place it on a higher elevation, you transferred energy to it and gave it potential energy. The potential energy is transformed into kinetic energy when the object falls down, and if it hits a surface, the energy will scatter, vibrating the areas around it and producing sound.
Also, work= force X distance. The energy does not go away, but rather get changed into some other form of energy

Take an object on the ground. Now lift it up. You do work against gravity. Now, if you let it drop then it'll gain speed as it falls. That;s kinetic energy. It came from the potential energy which was from the work done. So, work went to potential energy and now goes to kinetic energy.

Alinear relationship would look like a straight line on the graph. it could have a positive negative or 0 slope. so it could be a line potentially going in any direction but it must be perfectly straight.