Which structure contains hereditary material and codes for all characteristics of an organism?

The chromosome is the correct choice because it serves as a tightly packed structure that contains DNA, which holds the hereditary material. Chromosomes are found in the nucleus of eukaryotic cells and are crucial for the proper distribution of genetic information during cell division. Each chromosome is made up of a long DNA molecule that contains many genes, which are the specific sequences that code for traits and characteristics of an organism.

In this context, while genes are specific sequences within the DNA that determine particular traits, and a DNA strand refers to the long molecule itself, chromosomes encompass the entire structure needed for an organism's genetic inheritance. RNA molecules, on the other hand, play roles in coding, decoding, regulation, and expression of genes but do not carry hereditary material in the same way that chromosomes do. Thus, for the assertion that structure holds hereditary material and codes for all characteristics, chromosomes are the most comprehensive answer.