Site of respiration in bacteria is the "mesosome".

Explanation:
The mesosome is a specialized invagination of the plasma membrane in bacteria. It plays a crucial role in various cellular processes, including respiration.

1. Role of the mesosome in respiration:
- The mesosome is involved in the electron transport chain, which is an essential part of aerobic respiration in bacteria.
- It contains enzymes and electron carriers necessary for the oxidation of nutrients and the production of ATP.
- The electron transport chain occurs on the inner surface of the mesosome, where the plasma membrane folds inwards.
- These invaginations increase the surface area of the membrane, allowing for more respiratory enzymes and electron carriers to be present, enhancing the efficiency of respiration.

2. Other functions of the mesosome:
- The mesosome is also involved in other cellular processes, such as DNA replication, cell division, and secretion.
- It helps in the segregation of DNA during cell division by attaching to the replicating DNA and pulling it apart.
- The mesosome is also implicated in the synthesis and secretion of various substances.
- It provides an attachment site for ribosomes, which are involved in protein synthesis.

3. Comparison with other options:
a) Episome: Episomes are extrachromosomal genetic elements that can integrate into the bacterial chromosome. They are not directly related to respiration.
b) Ribosome: Ribosomes are cellular structures involved in protein synthesis. Although they may associate with the mesosome, they are not the site of respiration.
d) Microsome: Microsomes are artificial vesicles derived from the endoplasmic reticulum of eukaryotic cells. They are not present in bacteria and are unrelated to bacterial respiration.

In conclusion, the correct option is "c) mesosome" as it is the specialized invagination of the plasma membrane in bacteria that serves as the site of respiration.