The function \( n(t) = 50(1.2)^t \) represents the number of bacteria at time \( t \) in hours. In this experiment, the scientist is observing the population of bacteria from \( t = 0 \) hours (when there are 50 bacteria) and checks it specifically at \( t = 4 \) hours.
Given that the scientist is only considering the bacteria growth up to 4 hours, the most relevant and appropriate domain for \( t \) would be from 0 to 4 hours.
Thus, the correct option for the appropriate domain of the problem is:
Option #3: \( 0 \leq t \leq 4 \).