Concept:Cell sap is the liquid medium through which substances are moved within plant vascular tissues.Explanation:Cell sap is found inside the vacuoles of plant cells.It consists mainly of water and dissolved minerals, sugars, and other substances.In vascular tissues, xylem vessels and phloem sieve tubes rely on this fluid to transport water and nutrients across the plant.The nucleus controls cell activities, mitochondria generate energy, and ribosomes make proteins; none of these act as a transport medium.Answer:A. cell sap