To turn on an n-channel enhancement mode MOSFET, a positive voltage must be applied to the gate terminal relative to the source. This positive gate voltage induces a strong electric field that attracts electrons towards the gate, forming an n-type channel between the source and drain. Here is the step-by-step explanation:

1. In an enhancement mode MOSFET, the device is normally off when there is no gate voltage applied. The gate-to-source voltage needs to be increased to a positive value above a certain threshold voltage to create the conduction channel.
2. The gate of an n-channel MOSFET is usually made of a material like polysilicon, separated from the channel by a thin silicon dioxide layer. This configuration makes the gate an insulated gate.
3. When a positive voltage is applied to the gate relative to the source, it establishes an electric field across the oxide layer.
4. This field induces a channel of mobile electrons in the p-type substrate beneath the gate, allowing current to flow from the drain to the source when a voltage is applied across them.
5. Options such as 'a zero voltage' or 'a negative voltage' would not create the necessary channel for conduction in an n-channel enhancement MOSFET. 'A gate current' does not apply since the gate current in a MOSFET (ideally) is zero due to the insulating oxide layer.

Therefore, the correct answer is: A positive voltage.