Which proximal surface of a tooth is closest toward the midline of the body?

The mesial surface of a tooth is the one that is closest to the midline of the dental arch, which is an imaginary line that divides the body into right and left halves. In dental terminology, "mesial" specifically refers to the surface of a tooth that faces toward this midline. This is critical in understanding tooth arrangement and positioning; for example, in a typical dental arch, the first molar's mesial surface is closer to the midline than any other surface of that tooth.

In contrast, the occlusal surface refers to the top surface of the tooth where chewing occurs, the facial surface pertains to the side of the tooth that is visible in the front of the mouth (towards the lips or cheeks), and the incisal surface refers to the cutting edge of anterior teeth. None of these options are oriented towards the midline in the same way that the mesial surface is, making the mesial the definitive answer in this context.