**Core Concept**
Phosphofructokinase (PFK) is a crucial enzyme in the glycolytic pathway that catalyzes the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate. This enzyme requires various cofactors and ions to facilitate its activity.

**Why the Correct Answer is Right**
Phosphofructokinase requires magnesium ions (Mg²⁺) as a cofactor for its activity. Magnesium plays a crucial role in stabilizing the enzyme's active site and facilitating the transfer of phosphate groups during the reaction. In the absence of magnesium, PFK's activity is significantly impaired. The magnesium ion also helps to maintain the enzyme's conformation, ensuring proper substrate binding and product release.

**Why Each Wrong Option is Incorrect**
**Option B:** Inorganic phosphate is not a required cofactor for phosphofructokinase activity. While phosphate groups are involved in the reaction, inorganic phosphate is not necessary for the enzyme's catalytic function.
**Option C:** Manganese ions (Mn²⁺) are not required by phosphofructokinase. Although manganese can substitute for magnesium in some enzymatic reactions, it is not essential for PFK's activity.
**Option D:** Copper ions (Cu²⁺) are not required by phosphofructokinase. Copper is involved in various enzymatic reactions, particularly those related to redox processes, but it is not necessary for the glycolytic pathway.

**Clinical Pearl / High-Yield Fact**
Magnesium deficiency can impair glycolytic flux, leading to increased lactate production and potentially causing metabolic acidosis. This is particularly relevant in critically ill patients or those with chronic magnesium deficiency.

**✓ Correct Answer: A. Magnesium**