Q.The gaseous envelope around the earth is known as atmosphere. The region lying between an altitudes of 11-50 km is __________

Smog is a ground-level air-pollution phenomenon, and all ground-level weather/pollution activity happens in the lowest atmospheric layer -- the troposphere.

The troposphere is the atmospheric layer closest to Earth's surface (up to about 10-12 km altitude), where nearly all weather, clouds, and human-generated air pollution -- including smog (a mix of smoke, particulate matter, and photochemical pollutants like ozone and NOx formed from vehicle/industrial emissions reacting in sunlight) -- occur.

The stratosphere, above the troposphere, is where the protective ozone layer resides; it is not where ground-level smog forms.

Honesty note: this fact belongs to the Environmental Chemistry chapter, which does not appear (or have a close substitute) in the current rationalised syllabus menu provided for this platform -- flagged out-of-syllabus.