Final answer:
The synonym for 'utilize' is 'use'. This is a three-letter word that conveys the same meaning of making practical and effective use of something.
Step-by-step explanation:
The three-letter word that means the same thing as "utilize" is use. In the context of the options provided, 'act' refers to the process of taking action, 'run' is associated with moving at a speed faster than a walk, and 'aim' pertains to directing a course toward a target. Thus, the correct answer is (a) Use, as it is a synonym for 'utilize' which means to make practical and effective use of something.