Which cellular component is also the site for transcription and replication in eukaryotic cells?

The nucleus is the correct choice as it serves as the primary site for both transcription and replication in eukaryotic cells. Transcription, the process of synthesizing RNA from a DNA template, occurs in the nucleus where the genetic material (DNA) is housed. The replication process, which involves copying the entire genome for cell division, also takes place within the nucleus.

In eukaryotic cells, the nucleus is enclosed by a double membrane called the nuclear envelope, which not only protects the DNA but also regulates the transport of molecules in and out of the nucleus. This compartmentalization is critical for the orderly processes of transcription and replication, as it allows for the necessary enzymes and factors to operate efficiently in a controlled environment.

The other options refer to components not involved in these processes: the nucleoid is characteristic of prokaryotic cells, where genetic material is located in a region not surrounded by a membrane, the capsule is associated with the outer structure of some bacteria and aids in protection and adherence, while ribosomes are the sites of protein synthesis, and do not carry out transcription or replication.