What is the nucleus of a bacterium known as?

In prokaryotes,the mesosome performs a function similar to that of a:

In the given figure,what do $a$ and $b$ represent respectively?

Which of the following are characteristics of prokaryotic cells?
$(a)$ Ribosomes are made of $50S$ and $30S$ subunits
$(b)$ They can have plasmids
$(c)$ They contain mesosomes
$(d)$ They have peroxisomes
Choose the correct answer from the options given below:

Which of the following inclusion bodies are absent in the cytoplasm of bacteria?

In bacteria,the mesosome is a specialized structure formed by the invagination of which component?