Use the drawing of a molecular orbital energy diagram for clf (assume that the σp orbitals are lower in energy than the π orbitals.) to predict the bond order?

The bond order of the ClF molecule have to get from the molecular orbital energy level diagram.

The bond order in ClF is 1 with 2 pairs of lone pair of electron on both the atom.

The energy level diagram of the molecule is shown in the figure.

There are 2 electrons in the bonding orbital i.e. σp orbital and no electron in its corresponding anti bonding orbital.

Now we know the bond order of a molecule is [Electrons in boding orbital - electrons in anti bonding orbital]

Thus in this case [2-0] = 1.

There remains 2 pairs of non bonding electrons on Cl and F atom as shown in the diagram. The energy level of 2p valence electron of F atom will be lower than the 3p valence electron of Cl atom.  

the answer is d. 74.02