The primary nerve responsible for breast innervation is the ____ intercostal nerve.

The correct answer reflects the fact that the 4th intercostal nerve plays a significant role in providing sensory innervation to the breast. The breast is primarily innervated by the intercostal nerves, and while several intercostal nerves contribute to its sensory supply, the 4th intercostal nerve is notably responsible for the region of the breast mostly involved in sensation.

The 4th intercostal nerve innervates the skin overlying the medial aspect of the breast and is crucial for relaying sensory information. Each intercostal nerve corresponds to specific dermatomes, with the 2nd to 6th intercostal nerves being the primary contributors to breast sensation. Given the primary focus on the nerve that has the most significant effect on breast sensation and function, the 4th intercostal nerve is vital in this context, thus making it the correct choice.