If the distance binocular P.D. is 68 and the near P.D. is 64, the segment inset will be:

To determine the segment inset accurately, it's essential to understand the relationship between the distance and near pupillary distances (P.D.).

In this scenario, the distance binocular P.D. measures 68 mm, while the near P.D. is 64 mm. The segment inset refers to how far the segment of a multifocal lens should be inset from the distance measurement to align properly with the wearer's pupils when they are looking at near objects.

The inset calculation is based on the difference between the distance P.D. and the near P.D. Thus, to find the inset, you subtract the near P.D. from the distance P.D.:

68 mm (distance P.D.) - 64 mm (near P.D.) = 4 mm.

This value represents the total inset required. However, when calculating the inset for each lens, you would divide this total by two, as each lens is inset symmetrically from the optical center to the point where the near vision zone needs to be positioned over the pupil for optimal use. Thus, 4 mm total inset results in a 2 mm inset for each lens.

This rationale supports the conclusion that segment inset is 2 mm, confirming the answer.