The regulatory center of various activities in a cell is:

In the given figure,what does the part labeled '$a$' represent and what is its function?

During cell division,to which part of the chromosome do the spindle fibers attach?

Which of the following is associated with the synthesis of ribosomal $RNA$ $(rRNA)$?

The given diagram shows the types of chromosomes (labelled as $A, B, C$ and $D$) based on the position of the centromere. Which one is the correct option for the labelled chromosomes $A, B, C$ and $D$?

Which type of chromosome is represented by the given figure?