Final answer:
The correct molecular structure for NH4+ is tetrahedral, as it has four hydrogen atoms symmetrically arranged around a central nitrogen atom and no lone pairs.
Step-by-step explanation:
The correct molecular structure for NH4+ is tetrahedral. This is because in NH4+, which is also known as the ammonium ion, the nitrogen atom is at the center with four hydrogen atoms symmetrically arranged around it. There are no lone pairs on the nitrogen in NH4+, so the electron groups (the four N-H bonds) are oriented in the shape of a tetrahedron as well. Since all four electron groups are bonding pairs, the molecular structure is tetrahedral, matching the electron-pair geometry. This is in contrast to NH3 (ammonia), which has a trigonal pyramidal shape because of the presence of one lone pair.