Answer:
a. At pH 9, the product, p-nitrophenol, will be ionized, the solution will appear yellow in color, and thus can be monitored at the wavelength of maximum absorption for the phenolate ion which is 400nm
Explanation:
In alkaline phosphatase assay, the hydrolysis of p nitrophenyl phosphate to p nitrophenol happens. When the ph is 9, the product which is p nitrophenol would undergo ionization. The solution is going to appear to be of yellow and it can be monitored at a wavelength for maximum absorption of phenolate ions at 400nm.
Option A is the answer to the question.