Answer to No.1

a) Enhancement mode

An n-channel MOSFET transistor is Normally-OFF (enhancement mode) if its threshold voltage, VTn, is a positive value.  You can verify this by adjusting the VT and observing its value in the lower-right box.

• A MOSFET device is normally-OFF if the device is OFF at zero gate-source bias (Vgs = 0).
• An n-channel MOSFET is ON if Vgs is more positive than VTn (i.e., Vgs > VTn ).
• If VTn = 2.5 V (a positive voltage), then the NMOS is OFF because Vgs < VTn at Vgs = 0 V, and thus it is normally-OFF.