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 5.

To determine the most appropriate preposition to fill in blank number 5, let us analyze the sentence structure and word collocations:
The sentence is: "People also believe that yawning is associated (5) ________ boredom."

The verb phrase in question is "is associated". In the English language, the verb "associate" (when used in the passive voice as "be associated") is conventionally followed by the preposition "with" to connect two related things, concepts, or states.
Therefore, the standard collocation is associated with.

Let us look at why the other options are incorrect:
- "associated on" is grammatically incorrect and does not exist in standard English usage.
- "associated in" is only used in very specific, unrelated contexts (e.g., "associated in business" meaning partners in a business, but not to mean linked to or caused by).
- "associated by" is incorrect here because "by" indicates the agent performing an action, whereas here we want to show a relationship or connection.
- "associated with" correctly establishes the link between yawning and boredom.

Thus, the correct option to fill in blank 5 is with.