Question

Nucleus is absent in
A. Companion cells
B. Sieve tube cells
C. Phloem parenchyma
D. Cambium

Answer 1

Nucleus, is a specialized structure occurring in most cells and separated from the rest of the cell by a double layer, the nuclear membrane. The nucleus controls and regulates the activities of the cell and carries genes, structures that contain the hereditary information. Nucleoli are small bodies often seen within the nucleus. The gel-like matrix in which the nuclear components are suspended is the nucleoplasm.

Complete answer:
There are various functions of the nucleus.
The nucleus provides a site for genetic transcription that is segregated from the location of translation in the cytoplasm, allowing levels of gene regulation that are not available to prokaryotes.
The nucleus is an organelle found in eukaryotic cells. Inside its fully enclosed nuclear membrane, it contains the majority of the cell's genetic material. This material is organized as DNA molecules, along with a variety of proteins, to form chromosomes.
Let us discuss about each options in brief:
Companion cell : This is found within the phloem of flowering plants. Each companion cell is associated with a sieve element which functions in regulating activity of adjacent sieve elements and takes part in loading and unloading sugar into the sieve element. They have a nucleus.
Sieve tubes are columns of sieve-tube cells that have perforated, sieve-like areas in their lateral walls, which provide channels for food substances to travel. They are living cells which do not contain a nucleus.
Phloem parenchyma : There are two major types of complex permanent tissues: Xylem and phloem. Xylem is responsible for the conduction of water and minerals from the roots upwards to the shoots and leaves and phloem carries out the function of translocation. Phloem consists of sieve elements, companion cells, phloem sclerenchyma, and phloem parenchyma.
Phloem parenchyma comprises parenchyma cells. It has a thin cell wall, large vacuole, a prominent nucleus, and a protoplast.
Cambium : Cambium is a single layer of cells, which divide to produce secondary xylem cells toward the central axis of the stem and secondary phloem cells toward the outside. The cambium originates from undifferentiated cells that have retained their embryonic capacity for continued growth and differentiation.

Therefore, the correct answer is sieve tube cells.

Note: Other Functions:
RNA - In addition to DNA the nucleus holds another type of nucleic acid called RNA (ribonucleic acid). RNA plays an important role in making proteins called protein synthesis or translation.
DNA replication - The nucleus can make exact copies of its DNA.
Transcription - The nucleus makes RNA which can be used to carry messages and copies of DNA instructions.
Translation - The RNA is used to configure amino acids into special proteins for use in the cell.
Cell Nucleus:
The nucleus was the first of the cell organelles to be discovered by scientists.
It usually takes up about 10 percent of the cell's volume.
Each human cell contains around 6 feet of DNA which is tightly packed, but very organized with proteins.
The nuclear envelope breaks down during cell division, but reforms after the two cells separate.
Some scientists think that the nucleolus plays an important role in cell aging.
The cell nucleus was given its name by Scottish Botanist Robert Brown.