Explanation:
Polytene chromosomes are a unique type of chromosomes found in certain cells of organisms. They are characterized by multiple copies of condensed chromatin, which gives them a distinctive banded appearance under a microscope.

Chromocenter:
The point at which polytene chromosomes appear to be attached together is called the chromocenter. It is a dense region of chromatin where multiple polytene chromosomes are clustered or fused together.

Formation of Chromocenter:
During interphase, the DNA in the nucleus of a cell undergoes replication, resulting in the formation of multiple copies of each chromosome. In certain cells, such as those found in the salivary glands of insects, the chromosomes do not separate after replication. Instead, they align side by side and become tightly coiled, forming a structure known as a polytene chromosome. These polytene chromosomes are visible under a microscope, and they appear to be attached together at a specific point known as the chromocenter.

Function of Chromocenter:
The chromocenter plays a crucial role in the organization and regulation of gene expression within the cell. It helps to facilitate the coordination and regulation of gene activity by bringing together specific regions of different chromosomes that contain related genes. This clustering of genes allows for efficient gene regulation and coordination of gene expression.

Other Options:
- Centriole: Centrioles are small, cylindrical structures found near the nucleus of animal cells. They play a role in cell division and the formation of the spindle fibers during mitosis.
- Centromere: The centromere is the region of a chromosome where the two sister chromatids are held together. It is important for proper chromosome segregation during cell division.
- Chromomere: Chromomeres are bead-like structures observed along the length of polytene chromosomes. They represent areas of active gene transcription.

Conclusion:
The correct answer is option D, chromocenter. The chromocenter is the point at which polytene chromosomes appear to be attached together. It plays a crucial role in the organization and regulation of gene expression within the cell.