Answer:
6 different frequencies
Step-by-step explanation:
From energy level 1 to 2 is one frequency, from energy level 1 to 3 is one frequency and From energy level 1 to 4 is one frequency. So, we have a total of 3 frequencies for transition from energy level 1.
From energy level 2 to 3 is one frequency and from energy level 2 to 4 is one frequency. So, we have a total of 2 frequencies for transition from energy level 2.
From energy level 3 to 4 is one frequency.
So we have a total of 3 + 2 + 1 different frequencies = 6 different frequencies.
Note that the reverse process for each step produces the same frequency as the step in consideration.