What does a spherometer measure?
A spherometer is used to measure:

• the radius of curvature of spherical surfaces,
• the thickness of thin plates/sheets.

It works by measuring a small height/depth difference $h$ relative to a reference plane.

Check each option

A: Radius of curvature of concave surface
A spherometer can measure the sag/depth of a concave spherical surface, so its radius of curvature can be found.
Hence, A can be measured.

B: Specific rotation of liquids
Specific rotation is an optical property measured using a polarimeter, not a spherometer.
Hence, B cannot be measured.

C: Thickness of thin plates
A spherometer is commonly used to measure the thickness of a thin plate by comparing readings with and without the plate.
Hence, C can be measured.

D: Radius of curvature of convex surface
A spherometer can also measure the sag of a convex spherical surface and hence determine its radius of curvature.
Hence, D can be measured.

Conclusion

The physical quantity that cannot be measured using a spherometer is: $\boxed{\text{Specific rotation of liquids}}$

So the correct option is B.