Which term describes a succession of letters that represent the same phonological unit?

The correct term that describes a succession of letters representing the same phonological unit is "phonogram." A phonogram consists of a letter or a group of letters that correspond to a particular sound or group of sounds in spoken language. For example, "at" in the words "cat," "hat," and "bat" illustrates how a series of letters associates with a consistent pronunciation or phonological pattern.

In contrast, a phoneme refers to the smallest unit of sound in speech that can distinguish one word from another, without necessarily focusing on letter representation. A morpheme is the smallest meaningful unit of language, and can either be a word or a part of a word, such as a prefix or suffix, but does not specifically pertain to successions of letters representing sound. A syllable is a unit of speech that includes a vowel sound or a syllable nucleus but does not focus on the letter representation aspect as a phonogram does. Thus, the emphasis on letters that align with a specific phonological unit makes "phonogram" the appropriate choice.