The correct option is C Assertion is correct but Reason is incorrect.
Phloem tracheids transport not only sugars, but also other organic compounds like nitrogenous compounds. The transport in phloem tracheids is not dependent on concentration of individual substances but occurs in mass flow depending on pressure gradient between source and sink. In this mass flow all the organic substances flow in a mass irrespective of the individual concentration gradients of each of them. As nitrogen is an important mineral in order to conserve nitrogen, nitrogen is transported from senescing and dying parts of plants as old leaves to the younger growing parts.