The correct answer is: A. Source to drain.
In a MOSFET, electrical charge flows from the source to the drain. The source is the terminal where the charge carriers enter the device, and the drain is the terminal where they leave. The gate is a control electrode that modulates the flow of charge between the source and drain.
The other options are incorrect because they do not represent the correct flow of charge in a MOSFET. Option B, drain to source, is the opposite of the correct flow of charge. Option C, source to ground, is not a valid option because the ground is not a terminal of a MOSFET. Option D, source to gate, is not a valid option because the gate is a control electrode, not a terminal.
I hope this helps!