The rate of reaction involving ions can be studied by _ method.

Explanation

A is incorrect as a dilatometer is a scientific instrument that measures volume changes caused by a physical or chemical process.B is incorrect as refractometry is an optical analytical technique used to establish the refractive index of transparent and translucent substances. The method provides information about the substance's chemical composition, purity, and physical properties and is commonly used as a quality control method.C is incorrect as optical rotation, also known as polarization rotation or circular birefringence, is the rotation of the orientation of the plane of polarization about the optical axis of linearly polarized light as it travels through certain materials.D is correct as electrical conductivity tells us how well a material will allow electricity to travel through it.