Before experimenting with the pendulum task apparatus, a teenager is asked which variable determines the rate of swinging, and she says, "weight." the teen then systematically tests this hypothesis by testing one variable at a time while holding the other variables constant in order to get the right answer. this teen is most likely in the stage.

formal operations

Explanation:

In the period of formal operations the young man is already able to reason about hypotheses insofar as he is able to form abstract conceptual schemes and through them to perform mental operations within the principles of formal logic. With this, the adolescent acquires "the capacity to criticize social systems and propose new codes of conduct: discusses moral values of their parents and builds their own (thus acquiring autonomy)".

An example of the period of formal operations can be seen in the question above, where a teenager was able to think for himself how to test the hypotheses of an experiment to arrive at a concrete conclusion about it.

i believe the answer would be:

d. violin, clarinet, french horn, and tuba

b.a device that inputs heat into a cool object