The exosphere is the layer of the atmosphere a. closest to the earth's surface. b. that absorbs ultraviolet radiation from the sun. c. where gas molecules can be exchanged between earth's atmosphere and outer space. d. where air temperature stops increasing with altitude and starts decreasing.
You can read in wikipedia that in planets with substantial atmposphere, the exosphere is the uppermost layer of the atmosphere where it merges with the interplanetary space (credit to wikipedia). Then c. is the right option.