Answer:
Mars, the red planet, orbits the sun.
Step-by-step explanation:
An appositive is a noun or a noun phrase that renames the noun or subject in the sentence. In other words, appositives are used to describe or write something about the noun in the sentence.
The given sentence "Mars orbits the sun" can be rewritten as "Mars, the red planet, orbits the sun". Here, the phrase "the red planet" is an appositive and describes or says something about the noun/ subject "Mars".
Thus, the appositive is "the red planet".