This question relates to the Valence Shell Electron Pair Repulsion (VSEPR) theory, which predicts molecular geometry based on minimising repulsion between electron pairs (both bonding and lone pairs) around a central atom.

For a central atom with sp³ hybridisation and four electron domains arranged with no lone pairs (e.g., methane, CH₄), the electron pairs arrange themselves symmetrically at the vertices of a regular tetrahedron, giving the ideal bond angle of 109.5°.

However, when one of those four hybrid orbitals holds a lone pair instead of a bonding pair (as in ammonia, NH₃), the geometry is distorted. VSEPR theory ranks the strength of repulsive interactions as:

lone pair–lone pair > lone pair–bond pair > bond pair–bond pair

This ordering arises because a lone pair is held only by the central atom's nucleus and is not "pulled" outward by a second bonded nucleus, so its electron cloud is more diffuse and occupies more angular space close to the central atom than a bonding pair does. Consequently, the lone pair pushes the neighbouring bond pairs closer together, compressing the angle between them.

For NH₃, which has three N–H bond pairs and one lone pair, this lone pair–bond pair repulsion squeezes the H–N–H bond angle down from the ideal 109.5° to approximately 107°. This confirms that the bond angle decreases due to lone pair–bond pair repulsion, which is the correct explanation.

The remaining options do not hold up: the angle does not increase, since a lone pair repels neighbouring bonds rather than allowing them to spread further apart; the geometry is not unchanged, precisely because introducing a lone pair breaks the perfect symmetry of the parent tetrahedral arrangement; and "increased nucleus attraction" is not the relevant mechanism at all — bond angle changes in VSEPR theory are explained through electron pair repulsion, not through nuclear attraction forces. As a further illustration of this trend, each additional lone pair compresses the angle further: water (H₂O), with two lone pairs, has an even smaller bond angle of about 104.5°.