Accumulation mode in an n-MOSFET refers to the condition when the gate voltage is negative relative to the source voltage. In this mode, the channel of the MOSFET is formed by the accumulation of majority charge carriers (electrons) in the channel region. Let's discuss each option to understand why option 'C' is the correct answer.

a) Gate is applied with positive voltage:
When the gate voltage is positive relative to the source voltage, it creates an electric field that attracts majority charge carriers (electrons) towards the gate. As a result, the channel becomes depleted of charge carriers, and the MOSFET operates in the depletion mode, not accumulation mode.

b) Gate is grounded:
When the gate is grounded, the gate voltage is zero. In this case, the MOSFET is in the off state, and no current flows through the channel. Therefore, the MOSFET is not operating in any mode.

c) Gate is applied with negative voltage:
When the gate voltage is negative relative to the source voltage, it repels majority charge carriers (electrons) away from the gate and creates an electric field that attracts minority charge carriers (holes) towards the gate. As a result, an accumulation layer of minority carriers is formed in the channel, and the MOSFET operates in the accumulation mode.

d) Gate is connected to the source:
When the gate is connected to the source, it forms a short circuit between the gate and source terminals. In this case, the MOSFET is in the off state, and no current flows through the channel. Therefore, the MOSFET is not operating in any mode.

In summary, the n-MOSFET operates in the accumulation mode when the gate is applied with a negative voltage relative to the source. This negative voltage repels majority charge carriers (electrons) and attracts minority charge carriers (holes), resulting in the formation of an accumulation layer in the channel.