Question

A scientist was observing an unknown slide under an electron microsocope; he found a cell without a well-defined nucleus but with a thick cell wall and ribosomes. What type of cell could it be?

Answer 1

Eukaryotic cell

There are differences between Eukaryotic and Prokaryotic cells. This difference is considered to be the most important distinction between groups of organisms. A Prokaryotic cell does not contain a nucleus. It only contains one chromosome and is a single-celled organism. It was the only form of life on earth for millions of years. Examples of a Prokaryotic cell are the different types of bacteria present today. 
A Eukaryotic cell contains a nucleus; more than one chromosome and is typically a multi-celled organism.  Both plant and animal cells are eukaryotic cells.