Answer:
Rocks
Step-by-step explanation:
Rocks because those 4 planets Mercury, Venus, Earth, and Mars are rocky planets. Also, this is a tip, anytime you are forgetting the order of the planets, use this term:
My
Very
Excellent
Mother
Just
Served
Us
Nachos
It will help you remember the actual order which is
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune