Mathew would observe that the light would burn the brightest in the strongest acid.
Explanation:
As Mathew is using conductivity apparatus to measure the effect of concentration of hydrochloric acid in lab. As it is known that hydrochloric acid can increase the intensity of light burning in the lab when conductivity test is done.
So the light would be burning brighter on increasing the concentration of hydrochloric acid. When the concentration reaches the maximum point that is when the hydrochloric acid is the strongest in its concentration at this time, the light would burn the brightest. This would be observed by Mathew.