Nucleoli were discovered by

The correct option/answer is Felice Fontana.

Here is a step-by-step educational explanation of the discovery of the nucleolus:

1. Definition of the Nucleolus:
The nucleolus is a distinct, dense sub-nuclear structure found within the nucleus of eukaryotic cells. It is not membrane-bound and plays a primary role in the transcription and assembly of ribosomal RNA (rRNA).

2. Historical Discovery:
The nucleolus was first observed and described by the Italian scientist Felice Fontana in the year 1781. Fontana observed this structure in the slime of an eel's skin, making it one of the earliest documented observations of an intracellular organelle.

3. Analysis of other options to clarify:
• Leeuwenhoek (Antonie van Leeuwenhoek) is widely known as the "Father of Microbiology" for his pioneering work in microscopy and his discovery of single-celled organisms (microorganisms).
• Robert Hook (Robert Hooke) first discovered and coined the term "cell" in 1665 while observing cork tissue under a microscope.
• Robert Brown discovered and described the cell nucleus in 1831.

Therefore, the credit for the discovery of the nucleolus goes directly to Felice Fontana.