The arrow in the symbol of a transistor indicates the direction of ___________

The other options are incorrect because:

• Option B: Electron current in the collector is in the opposite direction of the arrow.
• Option C: Hole current is the flow of positive charge carriers, which are not present in a transistor.
• Option D: Donor ion current is the flow of positive charge carriers due to the movement of donor impurity atoms in the semiconductor material. This is not the main current in a transistor.