13) F

1 – Wrong
Transmitter molecules diffuse to the receptors to be recognised. They are not formed in the receptors.

2 – Wrong
The signal is transmitted across the synapse by diffusion

3 – Wrong
This statement does not even make sense. Transmitter molecules are released before the signal is transmitted across the synapse.

4 – Correct
It is an impulse which causes the transmitter molecules to be released.

5 – Correct
The signal is transmitted across the synapse by diffusion.

Hence, 4 and 5 is correct.