A synonym is a word ______ a meaning similar to another word.

The correct answer is "Having" because this word serves as a present participle, which fits the context of the sentence. In this case, "having" indicates the possession of a quality, in this instance, a similarity in meaning. The phrase can be read fluently as "A synonym is a word having a meaning similar to another word," conveying the intended relationship concisely and correctly.

The other choices do not fit grammatically or contextually. "Who have" and "That have" do not appropriately relate to "word," as "word" is a singular noun, and thus these phrases lose the agreement. "Who is having" is also awkward and incorrect in this context, as it introduces a continuous aspect that does not align with the static nature of definitions. Overall, "Having" provides a clear and grammatically accurate completion to the sentence.