Which one occurs in an insignificant amount in xylem sap?
A. Water
B. Nitrate
C. Phosphate
D. Sugar

Hint::
 Plants make their food in the leaves while the roots take up the water from the soil. But the food and water are needed by the whole body of the plant, not only leaves or roots respectively. So they have various cells and systems to regulate the whole food-water to each part of the plant. Some tissues are dead while some are living.

Complete step by step answer:
Xylem is a vascular tissue which has the role of the conduction of water and minerals from the roots to the shoots in the plants. They help to make the woody stem. It also provides physical support. They have specialized cells called treachery elements. It is found in the roots. Stems and leaves of the plants.
Phloem is the tissue of the plant which transports the soluble organic substances to the various parts of the plant. This substance includes the sugars made by the photosynthesis process. They are usually found outside the xylem.
Option A: Water: Xylem transports mainly water throughout the plant.
Option B: Nitrate: It is a mineral transported by xylem.
Option C: Phosphate: It is a mineral transported by xylem.
Option D: Sugar: It is carried by the phloem from the leaves to the other parts.
So the answer is option D: Sugar.

Note:
Xylem is the dead tissue as most of the cells in it are dead except for the xylem parenchyma cells. Xylem and phloem are together called complex tissues as they have many cells carrying out the functionalities. They all work as a unit and in coordination. There are different functions to be performed by them.