In an n-type semiconductor, the majority carriers are electrons, which are supplied by donor impurity atoms. The energy levels associated with these donor impurities lie just below the conduction band.

The Fermi level represents the highest energy level occupied by electrons at absolute zero temperature. In an intrinsic semiconductor, the Fermi level lies in the middle of the bandgap between the valence band and the conduction band. However, when impurities are added to the semiconductor, the position of the Fermi level shifts due to the presence of additional energy levels introduced by the impurity atoms.

To understand the position of the Fermi level in an n-type semiconductor, we need to consider the energy levels associated with the impurity atoms. In the case of n-type semiconductors, donor impurity atoms, such as phosphorus or arsenic, are added. These impurity atoms have extra valence electrons compared to the host semiconductor atoms.

Now let's discuss the options given:

a) Donor band: Donor impurity atoms introduce energy levels just below the conduction band. These energy levels are called donor levels or donor bands. Electrons from the donor levels can easily jump to the conduction band, making them the majority carriers in an n-type semiconductor. Therefore, the donor band is just above the intrinsic Fermi level in an n-type semiconductor.

b) Valence band: The valence band is the highest energy band that is completely filled with electrons at absolute zero temperature. It is below the Fermi level in both intrinsic and extrinsic semiconductors.

c) Acceptor band: Acceptor impurity atoms introduce energy levels just above the valence band. These energy levels are called acceptor levels or acceptor bands. Electrons from the valence band can easily jump to the acceptor levels, leaving behind holes in the valence band. Acceptor levels are associated with p-type semiconductors, not n-type semiconductors.

d) Conduction band: The conduction band is the highest energy band that is partially filled with electrons at absolute zero temperature. It is above the Fermi level in both intrinsic and extrinsic semiconductors.

Therefore, the correct answer is option 'a' - Donor band.