Which process is happening in the reaction that is shown

The correct option is A.

Explanation:

Dehydration reaction is a type of chemical reaction that involves loss of water molecules. This type of reaction usually occur when chemical monomers combine with one another via covalent bond to form single, bigger molecules, molecules of water are usually released as by product.

If you examine the chemical structures given in the question, you will observe that the two reactants involved reacted together to form a larger molecule and a water molecule (H2O) was released in the process.  

NOTE: Always remember that DEHYDRATION means removal of water molecules.

bacterium is not correct