Which characteristic is noted as the only stable feature of letters in language?

The correct answer highlights that the name of a letter is the only stable feature in language. This stability is evident because regardless of the font, style, or language in which a letter appears, its name remains consistent. For example, the letter "A" is always referred to as "A," no matter if it's printed in uppercase, lowercase, or a different type of script.

While the shape of letters can vary significantly between different fonts or styles, their names do not change. Similarly, the sound associated with each letter can differ depending on context and language; for instance, the letter "C" can sound like /s/ in "circle" and /k/ in "cat." The tactile sensation of how a letter feels when written or typed can also vary and thus is not a reliable characteristic across different instances of letter usage. Therefore, the name is the singular feature that provides stability in the understanding and identification of letters across varying contexts.