Answer: Third Option; Something that is added to another thing
Step-by-step explanation:
In the dictionary the word adjunct refers to "a thing added to something else as a supplementary rather than an essential part."
This is how you would use it in a sentence "I was part of the adjunct jury, in the event that an actual member was dismissed.
When you read your options it is clear that the third option is the one that best fits the meaning to the definition.