Identify the physical quantity that cannot be measured using spherometer :

A) Radius of curvature of concave surface
A spherometer is specifically designed to measure radius of curvature (both concave and convex).
So this can be measured.

B) Specific rotation of liquids
This is an optical property measured using a polarimeter, not a spherometer.
Spherometer only measures mechanical dimensions (height, curvature).
So this cannot be measured.

C) Thickness of thin plates
Yes, a spherometer can measure very small thickness by comparing readings.
So this can be measured.

D) Radius of curvature of convex surface
Again, one of the main uses of a spherometer.
So this can be measured.

Final conclusion:
Only option B is not measurable using a spherometer.

Answer: Specific rotation of liquids