What type of malocclusion occurs when a tooth is not properly aligned with its opposing tooth?

The correct choice is crossbite. A crossbite occurs when one or more teeth are positioned either more towards the tongue or the cheek than their opposing teeth, leading to misalignment between the upper and lower arches. This can happen with individual teeth or groups of teeth and can affect both the anterior and posterior teeth. Crossbites can lead to issues such as uneven wear, tooth movement, and discomfort, ultimately affecting the bite's overall function.

In contrast, overbite refers to a condition where the upper front teeth excessively overlap the lower front teeth vertically. Mesiocclusion is a term typically associated with a specific type of dental occlusion, often an Angle Class II malocclusion, which signifies a more severe misalignment of the bite but is not solely focused on the individual tooth relationship. Crowding relates to the insufficient space in the dental arch for the teeth, resulting in teeth overlapping or being pushed together, rather than being about the alignment of opposing teeth. Each of these conditions affects occlusion in different ways, but crossbite specifically addresses the improper alignment between opposing teeth, making it the appropriate answer.