Which two elements make up the greatest
percentages by mass in earth’s crust?
(1) oxygen and potassium
(2) oxygen and silicon
(3) aluminum and potassium
(4) aluminum and silicon

The two most abundant elements in Earth's Crust are: 
2) Oxygen and silicon.

Oxygen comprises 46% and silicon comprises 27% of the total mass of Earth's crust. 

There is a large gap to the third spot and beyond, with Aluminum, Iron, and Calcium making up 8, 5, and 3 percent, respectively.