The layer in the wall of the heart which contains a fluid that protects the heart from shocks and mechanical injuries is known as:

The correct option is A pericardium
The wall of the heart is made up of three layers- pericardium, myocardium and endocardium.
The pericardium is the outermost layer which is a two-layered sac having outer parietal pericardium and visceral pericardium. In between the parietal and visceral pericardium, there is a cavity called the pericardial cavity. This cavity contains the pericardial fluid which protects the heart from shocks and mechanical injuries.

Myocardium and endocardium do not contain any fluid. The myocardium is the middle layer of the wall which is the thickest among the three layers and is made up of involuntary striated cardiac muscles. The endocardium is the innermost layer which is made up of simple squamous epithelium.