In earth's atmosphere, most gas particles are located in the
a. exosphere
b. mesosphere
c. stratosphere
d. troposphere

The correct answer is (c.) troposphere
The troposphere is the lowest portion of the atmosphere but also it is where most gas particles are located. It holds 75 percent of the atmosphere's mass. It is typically a hundred meters deep to 2 km. Above the troposphere is the tropopause, bordering between troposphere and the stratosphere. 

Answerland;