Final answer:
The expression t²-15t+54 is equivalent to (t-6)(t-9) and represents the minimum temperature reached by the substance.
Step-by-step explanation:
The expression t²-15t+54 can be factored as (t-6)(t-9), which is equivalent to Option A. In this expression, the minimum temperature reached by the substance occurs when t is equal to the average of 6 and 9, which is 7.5. Therefore, the minimum temperature reached by the substance is represented by Option A.