why enhancment type nMOS transistor is used as "pull up" in MOS invertor?

Question

in question insted of pull up it is "pull down"

Answer 1

For which technology? A purely NMOS process would have nothing but enhancement and depletion transistors. Depletion transistors draw power when the output is low. An enhancement pullup could be shut off to conserve power.

Modern CMOS inverters use PMOS pullups. They provide much better response and rise times than either NMOS enhancement or depletion transistors.