Statement (I): Thicker sections of casting take longer to solidify than thinner sections.

Statement (II): Thicker sections of casting carry residual stresses. 

Concept:

Time taken for solidification of casting is given by Chovrinov rule,

Thicker sections take longer time for solidification because they have higher V/A. This creates the differential temperature gradient and residual stresses are developed during the solidification process due to the temperature gradients between different parts of casting or due to the mechanical constraints imposed by the mould during shrinkage of the cast metal.

Therefore both the statements are individually true, and statement (II) is not the correct explanation of statement (I).