Which of the following is made of dead cells?

The correct option is Phellem.

To understand why phellem consists of dead cells, let us break down the structure and function of the plant tissues mentioned in the options:
1. Phellem (Cork): Phellem is formed toward the outer side by the activity of the cork cambium (phellogen). As these cells mature, their cell walls become heavily impregnated with a waxy substance called suberin. Suberin makes the cell walls impermeable to water and gases, which cuts off the cells from nutrients and water, eventually causing them to die. Therefore, mature phellem cells are dead at maturity and function as a protective outer barrier for the plant.
2. Phloem: Except for phloem fibres, other components of phloem (sieve tube elements, companion cells, and phloem parenchyma) are living cells.
3. Collenchyma: This is a simple permanent tissue composed of living cells with localized cell wall thickenings made of cellulose, hemicellulose, and pectin. It provides mechanical support to growing parts of the plant.
4. Xylem parenchyma: Xylem consists of both dead and living cells. Xylem vessels, tracheids, and xylem fibres are dead, but xylem parenchyma cells are living cells involved in storage and radial conduction of water.

Thus, among the given choices, phellem is the tissue that is made entirely of dead cells.