the issue comes from your code, it returns 'a' for last('a', 'b', 'c')

I passed that can you provide some code?

I guess:
the 4th and 5th function should return arr, not the length of arr, are you stuck here?

Test for: arr=[3,5,6,17,0,15] 
Test Passed: Value == 6
Test Passed: Value == 3
Test Passed: Value == 15
Test Passed: Value == 7
Test Passed: Value == [3,5,6,17,0]
Test for: arr=[12,5,18,15,4,15,1] 
Test Passed: Value == 7
Test Passed: Value == 12
Test Passed: Value == 1
Test Passed: Value == 8
Test Passed: Value == [12,5,18,15,4,15]

I try it and this is some result of testcase. It seems to be running normally.
Can you paste your code here(mark it as a Spoiler)?