A sentence is a group of words that expresses a complete thought. It is a fundamental unit of communication and allows us to convey ideas, information, and emotions. Let's break down the answer and explore why it is the correct choice.

I. What is a sentence?
A sentence is a collection of words that come together to form a complete thought. It consists of a subject and a predicate, which contain the necessary components to convey information. A subject is the main noun or pronoun that the sentence is about, and a predicate is the verb or verb phrase that expresses an action or state of being.

II. Expressing a complete thought
A complete thought refers to a coherent idea or message that can stand alone and make sense on its own. A sentence must convey a complete thought to effectively communicate with others. It should have a clear subject and predicate that work together to express a specific meaning.

III. Examples
1. "The cat is sleeping." - This sentence has a subject ("the cat") and a predicate ("is sleeping"). It expresses a complete thought by conveying the information that the cat is currently sleeping.
2. "I love ice cream!" - In this sentence, the subject is "I" and the predicate is "love ice cream." It expresses a complete thought by conveying the emotion of love towards ice cream.

IV. Emotion and action
While emotions and actions are important aspects of communication, they are not sufficient to make a sentence complete on their own. Emotions can be expressed through sentences, but they require a subject and predicate to form a complete thought. Similarly, actions can be part of a sentence, but they need to be combined with a subject to form a complete thought.

V. Conclusion
In conclusion, a sentence is a group of words that expresses a complete thought. It is the basic unit of communication and consists of a subject and predicate. While emotions and actions are important elements of sentences, they alone cannot form a complete thought. Therefore, option A, "thought," is the correct answer.