What is the optimum pH for enzyme X and why?

Absorbance at Various pHTime

Time (sec)	pH 4	pH 5	pH 6	pH 7	pH 8	pH 9
30	0.003	0.025	0.055	0.146	0.005	0.004
60	0.009	0.109	0.152	0.300	0.015	0.008
90	0.0120	0.195	0.255	0.432	0.038	0.010
120	0.020	0.215	0.341	0.552	0.065	0.012
150	0.023	0.333	0.413	0.659	0.081	0.013
180	0.025	0.360	0.478	0.755	0.090	0.013

You just performed an experiment to study the effects of pH buffers on enzyme X. A spectrophotometer set at 450 nm was used to measure absorbance which is when the substrate is converted to its product. Therefore, higher absorbance values suggest higher enzymatic activity. If the experiment was conducted at the same temperature, what is the optimum pH for enzyme X and why?