The following sentence has been split into four segments. Identify the segment that contains a grammatical error.
The fresh air / in the garden relaxes / the minds of children as / the environment is cleaner.

Correct option is D

Option (d) contains an error.
The error lies in the use of the comparative adjective "cleaner" without a proper comparison marker such as "than."
In comparative constructions, an adjective like "cleaner" must be followed by "than" and a noun or clause to complete the comparison. In this sentence, no comparison is explicitly made. There's no "than..." clause to complete the comparison.​
Use the superlative or absolute form if no comparison is intended:
"The environment is clean." 
Or add a comparison:
"The environment is cleaner than the playground."