The small part of the nucleus where ribosomal subunit formation takes place

The small part of the nucleus where ribosomal subunit formation takes place is called the nucleolus. This structure is found in eukaryotic cells and takes up about 25 percent of the volume of the nucleus. This structure is said to be the brain of the nucleus. It is made up of ribobucleic acids or the RNA and proteins. Its primary function is to transcript the ribosomal RNA (rRNA) and to combine these with the proteins resulting to the formation of an incomplete ribosome.  Since it is closely related to a chromosomal matter of a cell and important in the production of ribosome, this structure is correlated to be one of the cause of a number of diseases.

The most plausible answer is b, duration. the given amount of time is considered how long it last for.