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The correct answer is lactic acid, which is crucial in understanding post-mortem muscle metabolism. After slaughter, muscle continues to undergo anaerobic respiration due to the depletion of oxygen. This anaerobic process converts glycogen stored in the muscle into energy, resulting in the production of lactic acid.

As lactic acid accumulates, it causes a decrease in the pH of the muscle tissue, leading to a condition known as post-mortem glycolysis. The drop in pH is significant for several reasons. It contributes to the meat's texture and flavor, influences the color, and is an important factor in meat preservation by reducing the growth of spoilage organisms.

The other acids listed have different roles and are not the primary contributor to the post-mortem pH drop. Acetic acid, for example, is a byproduct of fermentation and not a primary metabolite of muscle glycogen breakdown. Citric acid is part of the Krebs cycle and plays a role in cellular respiration but is not produced in significant quantities in post-mortem muscle. Formic acid, while it can be produced in some metabolic processes, is not relevant in this context. Thus, lactic acid is the key substance responsible for the acidity change in