Nucleus is absent in?

Hint: Nucleus is a membrane bound organelles that consists of genetic material mostly in Eukaryotic organisms it helps in the process of transcription and replication. It is one of the largest organelle inside the cell and takes about 1/10th of cell volume.

Complete answer:
The nucleus is usually not present in sieve tubes and mammalian erythrocytes.
Sieve tubes are usually found in flowering plants inside the phylum. They have a major role in transport of organic compounds which are formed by photosynthesis. These are the major conducting cells found in phloem. Cells without a proper nucleus are called prokaryotic cells.
Mature RBC does not have any nucleus and cell organelles in them. They usually degenerate when the RBC is fully matured.
Mature red blood cells do not have a nucleus and are called Red blood corpuscles because they do not have any nucleus.
The nucleus usually controls and regulates the activities of the cell in growth and cell metabolism. It carries the genes and structures that contain the hereditary information. Also, it controls the heredity characteristics of an organism.

Note:
Nucleoli are small bodies often seen within the nucleus. The gel-like matrix in which the nuclear components are suspended is the nucleoplasm.