Uhhhh Coulombic repulsion? Also, there are no attractive intermolecular forces to counterbalance the repulsion since ammonium has no net dipole nor any Lewis basic sites for H-bonding.
Agreed, don't over-complicate this: negative 'a' means repulsion, only like charges repel, all of the other choices are neutral so can neither attract nor repel. Two positive NH4+ molecules will repel each other.