To solve this problem, we need to determine which statement about the nucleolus is true. Let's analyze each option:$\newline \bullet$Option 1: It takes part in spindle formation.$\newline$ - The nucleolus is not involved in spindle formation. Spindle formation is primarily associated with the centrosomes and microtubules during cell division.$\newline \bullet$Option 2: It is a membrane-bound structure.$\newline$ - The nucleolus is not a membrane-bound structure. It is a dense region within the nucleus where ribosomal RNA (rRNA) synthesis occurs.$\newline \bullet$Option 3: Larger nucleoli are present in dividing cells.$\newline$ - Larger nucleoli are typically found in cells that are actively producing proteins, not necessarily dividing cells. Dividing cells may have smaller nucleoli as the nucleolus disassembles during mitosis.$\newline \bullet$Option 4: It is a site for active ribosomal RNA synthesis.$\newline$ - This statement is true. The nucleolus is the site where rRNA is synthesized and combined with proteins to form ribosomal subunits.$\newline$Therefore, the correct option is Option 4: It is a site for active ribosomal RNA synthesis.