Which of the following has a stable enzymatic activity at a PH of 1.8?

Pepsin has its optimum enzymatic activity at a pH between 1.8 and 2.0. It is remains stable, and still highly active, when the pH drops to as low as 1.0 (Ryle 1970). Pepsin will begin to lose activity around pH 5 and it becomes irreversibly inactive at a pH around 7. However, a high concentration of pepsin will not become inactive until a pH of about 8. The activity of pepsin is also dependent upon the enzyme to protein ratio. The higher this ratio is the more efficient the enzyme becomes. Ref: Ganong's Review of Medical Physiology, 22 nd Edition, Page 471,468; Determining the Specificity of Pepsin for Proteolytic Digestion By Melissa H. Palashoff, 2008, Page 23.