There is a unique cycle that is repeated when we raise a number to an exponent. Here is an example of 7,
See, the cycle is 1-7-9-3-1-7-9-3....
And the question is What is the reason for 7 to the power of 7 ending with 7 as it's units place?But here we see that 7^7 has no 7 at its units place but 7^5 has. Instead 7^7 has 3 at its units place.
Hence, the question is actually wrong. But if asked we can write this as the answer.
Hope it helps...!