The cylinder strength of the concretre is less than the cube strength because of
the difference in the shape of the cross section of the specimens
the difference in the slendeness ratio of the specimens
the friction between the concrete specimens and the steel plate of the testing machine
the cubes are tested without capping but the cylinders are tested with capping.
Sir/Madam,
As of my knowledge is concerned it is option c.... also it is because of friction the strength of cubes is greater than actual strength (Cylinder strength)