Final answer:
The molecule AB with three single bonds and one lone pair around the central atom A has a trigonal pyramidal molecular geometry, similar to that of ammonia (NH3).
Step-by-step explanation:
The expected molecular geometry for a generic molecule AB with three single bonds and one lone pair of electrons on the central A atom would be trigonal pyramidal.
This can be deduced from the concept that the electron pair geometry would be a tetrahedral arrangement when considering both the bonding pairs and the lone pair. However, because we are only focusing on the actual placement of the atoms and not the lone pairs, the molecule takes on a trigonal pyramidal shape. This situation is similar to that observed in ammonia (NH3), where the central nitrogen atom is surrounded by three hydrogen atoms and one lone pair.