| Error | Correction |
|-------|------------|
| less | fewer |
(less is used for uncountable nouns; fewer is used for countable nouns like books.)
The examiner looks for identifying less → fewer. "Books" is a countable noun, so fewer must be used. Less is reserved for uncountable nouns (e.g., less water, less time). This is a standard grammar error tested in CBSE board exams under error correction.