Of the choices that you have given here, the only one that makes any sense at all is A.  A is clearly the correct answer.

An alphabet is a system of writing in which each sound in a language is generally represented by a single symbol.  This is in contrast to other systems that have symbols for whole words (for example, the Japanese kanji) or that have symbols for syllables (the Japanese kana).

Fonts are typefaces; they are specific styles of printing the letters in an alphabet.  Clauses and phrases each consist of multiple words.  Therefore, none of B, C, or D could be correct.