The correct answer is D. NO.
The bond order of a molecule is a measure of the strength of the chemical bond between two atoms in the molecule. It is calculated by taking the difference between the number of bonding electrons and the number of antibonding electrons, and dividing by two.
In the case of NO, there are 10 valence electrons. There are 8 bonding electrons (2 from each of the two nitrogen-oxygen bonds) and 2 antibonding electrons (1 from each of the two nitrogen-oxygen bonds). Therefore, the bond order of NO is 2.5.
The other options are incorrect because they have different bond orders.
In HCl, there are 11 valence electrons. There are 1 bonding electron (from the hydrogen-chlorine bond) and 0 antibonding electrons. Therefore, the bond order of HCl is 1.
In CO, there are 10 valence electrons. There are 6 bonding electrons (2 from each of the two carbon-oxygen bonds) and 2 antibonding electrons (1 from each of the two carbon-oxygen bonds). Therefore, the bond order of CO is 2.
In F2, there are 18 valence electrons. There are 12 bonding electrons (6
from each of the two fluorine-fluorine bonds) and 0 antibonding electrons. Therefore, the bond order of F2 is 1.