Output the alphabet, the ALPHABET, or just a character

Pyth, 10 bytes

h/#z[GrG1z

Test suite

We start by constructing a list with 3 elements: the lowercase alphabet, the uppercase alphabet, and the input. ([GrG1z) Then, we filter this list on the number of appearances of the input in the elements being nonzero. (/#z) Finally, we take the first element of the filtered list.

TeaScript, 5 bytes

xN(0)

TeaScript has a (almost) built-in for this :D

Try it online (note: the online interpreter has been updated to TeaScript v3 so in which this is N0)

Try all the test cases

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TeaScript 3, 2 bytes [non-competing]

Using TeaScript 3, this can become 2-bytes. This is non-competing because TeaScript 3 was made after this challenge

N0

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1 byte alternative

If we could output 0123456789 for digits, then this could be:

°

LabVIEW, 23 LabVIEW Primitives

The selector (the ? on the cse structure) is connected to a vi that is called Lexical Class. It ouputs numbers from 1-6 depending on input, 5 is lower case 4 is upper case.

The for loop goes 26 times to create an alphabet or once to pass the symbol through.