If you wanted to know the letters found in the expression “the alphabet”, the WRITTEN form of that question should use quotation marks around that expression (as indeed the previous answer rewrote it).

In that case, my answer would NOT be to SPELL the words out for you, but to note which seven different letters you used, listing each just ONCE, and probably in alphabetical order [a standard way]: a, b, e, h, l, p, t.

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But you did NOT write it that way.

That leaves two possible meanings to your question:

1) The most natural interpretation is that you are asking for a list of the 26 letters included in the English alphabet. (To say”in the alphabet” is no less correct than “of the alphabet”. The only possible difference would be a minor matter of emphasis Saying “of” points us to the fact that these letters, together COMPRISE the alphabet, while “in” refers to the INCLUSION of each of these in the larger entity. That distinction might be important in some questions. Hard to see how it would be in this case. )

***Your answer, in that case is: a, b, c, d, e, f, g, h, i j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z

2) The other possibility (which ALSO might be expressed–BETTER expressed– with “of”) is that you are asking for a DEFINITION of what LETTERS are (In this case “in/of the alphabet” specifies what sort of “letters” you mean. The make clear that you are not referring to the letters I took out my mailbox today.)

For that I’d have to point you to a good dictionary entry. Based on the link below, I would answer,

“They are symbols or characters conventionally used in writing and printing to represent speech sounds.”

compare http://dictionary.reference.com/browse/letter

***Since as a rule the most natural reading should be adopted unless there are compelling reasons not to, I suggest that this is THE correct answer. In fact, the use of “in” helps make this understanding the MORE natural one (#2 should use “of”, or should be completely rephrased to make it’s point clear).

that means, you’re asking the letters in “the alphabet”.

so, it is T-H-E A-L-P-H-A-B-E-T

but if you’ve asked like this: what are the letters OF the alphabet?

that means simply the letters of the alphabet… from A – Z….

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

its as simple as that… 😉

a, l, p, h, a, b, e, t