The correct answer is: D. Allow free expansion.
An expansion joint is a structural joint that allows for movement between two adjacent parts of a structure. In the case of rigid pavements, expansion joints are used to allow for the expansion and contraction of the pavement due to changes in temperature. Without expansion joints, the pavement would be subjected to stresses that could cause it to crack.
Option A is incorrect because warping stresses are caused by the uneven heating and cooling of the pavement. Expansion joints do not relieve warping stresses.
Option B is incorrect because shrinkage stresses are caused by the drying and shrinking of the pavement. Expansion joints do not relieve shrinkage stresses.
Option C is incorrect because expansion joints do not resist stresses due to expansion. In fact, expansion joints allow for the free expansion of the pavement, which helps to relieve stresses.