The correct answer is option 'D', which states that the only plant cells without nuclei among the given options are sieve tubes. Let's understand why this is the correct answer.

Plant Cells and Nuclei:
Plant cells are the fundamental units of plants and possess various specialized structures that perform specific functions. One of the essential organelles found in plant cells is the nucleus. The nucleus is a membrane-bound organelle that contains the genetic material of the cell, including DNA and RNA. It plays a crucial role in regulating the activities of the cell, including protein synthesis, cell division, and gene expression.

Explanation of the Options:

a) Cambium Cells:
Cambium cells are meristematic cells found in the stem and root of a plant. They are responsible for secondary growth, which leads to an increase in the diameter of the stem or root. Cambium cells actively divide and differentiate into secondary xylem and phloem cells. These cells contain nuclei as they are actively involved in cellular processes.

b) Cells of Pericycle:
The pericycle is a layer of cells located just inside the endodermis in the root. It is responsible for lateral root formation and the initiation of secondary growth. The cells of the pericycle also contain nuclei, as they actively participate in various cellular processes such as cell division and differentiation.

c) Xylem Parenchyma:
Xylem parenchyma cells are a type of parenchyma cells found in the xylem tissue of plants. They are involved in storage and transportation of water and minerals. Xylem parenchyma cells, like other plant cells, contain nuclei. The nuclei are essential for the regulation of cellular activities and the synthesis of proteins and enzymes required for cell function.

d) Sieve Tubes:
Sieve tubes are specialized cells found in the phloem tissue of plants. They are responsible for the transport of organic substances, such as sugars, from the leaves to other parts of the plant. Sieve tubes are unique in that they lack nuclei at maturity. During their development, the sieve tube elements lose their nuclei, allowing for a more efficient flow of materials through the phloem. The absence of nuclei in sieve tubes is an adaptation to facilitate the rapid and uninterrupted transport of organic solutes.

Conclusion:
Among the given options, sieve tubes are the only plant cells without nuclei. The absence of nuclei in sieve tubes allows for more efficient movement of organic substances through the phloem. The other options - cambium cells, cells of pericycle, and xylem parenchyma cells - all contain nuclei, which are essential for their cellular functions.