At what position in the distillation apparatus does the temperature match the boiling point of the distilled substance?
The temperature in area A can well exceed B all right, especially if the distillation adapter is long or if there is a fractioning column or another condenser before the three-way adapter.
However, no target component is going to make it through the condenser unless its vapors reach the side arm (the one with the joint to condenser).

Apparently, it's only possible if the temperature of that area coincides with the boiling point of the target, that's why the thermocouple or thermometer probe is installed exactly at that area (B).

As you rightfully suggested the temperatures in areas C and D are arbitrary: it only should be above the m.p. to prevent clogging and less than b.p. so that the target is not evaporating away.

P. S. there is a vacuum adapter shown on the picture (area C), so there also might be distillation at lower pressure, which would also lower the b.p. of the mix constituents.
Since the question doesn't mention its usage, atmospheric pressure is assumed.