Assertion (A) : Phloem sap move from source to sink
Reason (R) : Hydrostatic pressure in sieve tube near source is high while hydrostatic pressure in phloem near sink is low. 

Assertion (A) is true. The movement of phloem sap from source to sink is driven by differences in hydrostatic pressure, as well as by the active transport of sugars. The source is typically a region in a plant where sugars are produced or loaded into the phloem, such as leaves, and the sink is a region where sugars are either used for growth or storage, such as roots, fruits, or storage organs.
Reason (R) is also true. At the source, where sugars are actively loaded into the sieve tubes of the phloem, there is an accumulation of sugars. This accumulation creates a high hydrostatic pressure, and the sugars, water, and other solutes in the phloem sap move towards areas of lower pressure, which is typically the sink. As the phloem sap moves toward the sink, the sugars are removed and used for various plant functions.

A and R correct, R explains A. Phloem sap move from source to sink because the pressure difference, created by the movement of sugars and water, drives the flow of phloem sap from source to sink.