Elongated or tube-like cells with thick and lignified walls and tapering ends are called tracheids.

Tracheids are one of the two types of conducting cells found in xylem tissues of plants. They are elongated cells with tapered ends and have thick secondary walls that are heavily lignified. These thick walls provide strength and support to the plant. Tracheids are dead cells at maturity and their end walls contain pits, which allow water and dissolved minerals to move laterally between adjacent cells.

Tracheids are involved in the transport of water and minerals within the plant. They play a crucial role in the process of water conduction from the roots to the aerial parts of the plant, including the stem, leaves, and flowers. They form a continuous network throughout the plant, connecting the roots to the rest of the plant.

Comparison with other options:

a) Xylem fibers: Xylem fibers are another type of cells found in xylem tissues. They are also elongated cells with thick secondary walls, but they do not have tapering ends like tracheids. Xylem fibers provide mechanical support to the plant and are not involved in water conduction.

b) Xylem parenchyma: Xylem parenchyma cells are living cells found in xylem tissues. They have thin cell walls and are involved in storage and lateral conduction of water and nutrients. They are not elongated or tube-like in shape.

c) Vessels: Vessels, also known as vessel elements, are the other type of conducting cells found in xylem tissues. They are elongated cells with open ends called perforation plates, which allow for efficient water conduction. Vessels have thinner cell walls compared to tracheids and are arranged end-to-end to form long tubes.

In conclusion, the correct answer is option 'D' - tracheids. Tracheids are elongated or tube-like cells with thick and lignified walls and tapering ends, and they are involved in water conduction within the plant.