Why do people yawn? No one seems to know the exact (1)________. Some people believe that we yawn (2) ________ we do not have enough oxygen in our bodies. In (3) ________ words, we yawn to breathe in (4) ________ oxygen. People also believe that yawning is associated (5) ________ boredom.

Select the most appropriate option to fill in blank number 1.

The correct option is reason.

Let us break down the logical explanation for choosing the correct word to fill in blank number 1:

1. Analyze the context: The paragraph begins with the question, "Why do people yawn?" The word "Why" directly asks for a cause, motive, or explanation behind the action of yawning.

2. Examine the sentence: The sentence reads, "No one seems to know the exact (1)________." We need a noun that fits naturally after the adjective "exact" and refers back to the cause of yawning.

3. Evaluate the options:
- reason: A cause, explanation, or justification for an action or event. This perfectly matches the question "Why..." because we look for a "reason" to explain "why" something happens. "Know the exact reason" is also a very common and natural English collocation.
- explanation: While similar in meaning, "exact reason" is more idiomatic in this specific context of addressing a simple "Why" question about a physiological function.
- basis: Refers to the foundation or starting point of something, which does not fit as well here.
- reality: Refers to the state of things as they actually exist, which is not what the question "Why" is asking for.

Therefore, "reason" is the most appropriate option to fill in blank number 1.