Final answer:
A positive lysine decarboxylase test after 48 hours appears purple, as the enzymatic activity results in alkaline byproducts, causing a pH-induced color shift in the media's indicator.
Step-by-step explanation:
A positive lysine decarboxylase test result after 48 hours would typically appear purple (D). This color change occurs because the reaction produces alkaline byproducts, which raise the pH of the media. When lysine is decarboxylated by the enzyme lysine decarboxylase, it generates cadaverine, a compound that increases the pH. A color indicator in the media, such as bromocresol purple, shifts from yellow at acidic pH to purple at alkaline pH, thus indicating a positive result for organism's capability to decarboxylate lysine.