There are 15 possible wavelengths.
Work Step by Step
To calculate the number of possible electron transitions, we'll consider the number of possible energy level transitions. This is the same as choosing two numbers from the range 1 to 6 and letting the higher number be the starting energy level and the lower be the ending. The number of ways to choose 2 numbers from a set of 6 is 15, so the answer is 15.