The one that cannot be further divided into smaller grammatically meaningful components is _________

The correct option is a morpheme.

In linguistics, a morpheme is defined as the smallest grammatical unit in a language. It is a meaningful unit of a language that cannot be further divided into smaller grammatically meaningful components without losing its meaning or changing into a non-grammatical element.

Let's analyze the given options to understand why this is the correct choice:
1. A phoneme: This is the smallest unit of sound that can distinguish one word from another in a language (e.g., the /p/ or /b/ sound in "pat" vs. "bat"). However, a phoneme itself carries no grammatical or semantic meaning.
2. A morpheme: This is the fundamental unit of grammar and meaning. For example, the word "unbreakable" consists of three morphemes: "un-" (prefix meaning not), "break" (root word), and "-able" (suffix meaning capable of being). None of these components can be broken down further into smaller meaningful grammatical pieces.
3. A diphthong: This refers to a sound formed by the combination of two vowels in a single syllable, in which the sound begins as one vowel and moves toward another (e.g., in "coin" or "loud"). Like phonemes, diphthongs deal with sound/phonetics, not grammatical meaning.
4. Syntax: Syntax refers to the set of rules, principles, and processes that govern the structure of sentences in a given language. It is a system of sentence construction, not a minimal component.

Therefore, the unit that cannot be further divided into smaller grammatically meaningful components is a morpheme.