The principal force driving movement in diffusion is the
temperature
particle size
concentration gradient
membrane surface area

Concentration gradient.

Explanation:

Diffusion may be defined as the process of the movement of solute particles down the concentration gradient. This process does not require the expenditure of energy.

The main driving force in diffusion is the concentration gradient. The molecules generally move from its region of the higher concentration towards the region its lower concentration.

Thus, the correct answer is option (3).