In this task, you are given a multiple-choice question about healthcare. Answer the question based on your information and classify your answers into '1', '2', '3', and '4'.

Question: If you want to estimate the effects of an intervention, you will use:
Options: <1> A transversal design. <2> A retrospective design <3> An ecological study. <4> A randomized clinical trial. <5> An observational design with random selection of the participants.
4