In each of the questions given below, four sentences have been given. One out of four sentences may or may not contain an error. Find out which of the following sentence has an error. If all the sentences are error-free then choose ‘ All are correct’ as your answer choice.

The correct answer is Option 3: "Nothing is ever constants and change will always be the order of the day."

Step-by-Step Explanation:
Let us analyze each sentence to identify the error:

1. "The students who went back home did not return."
This sentence is grammatically correct. The relative pronoun "who" correctly refers to "students", and the past tense verb "did not return" is correctly structured.

2. "The major reason behind pollution is stubble burning across the country."
This sentence is grammatically correct. The singular subject "reason" agrees with the singular verb "is".

3. "Nothing is ever constants and change will always be the order of the day."
This sentence contains a grammatical error. The word "ever" is an adverb modifying the adjective that describes "Nothing". Here, the plural noun "constants" is used incorrectly instead of the singular adjective "constant". The correct phrasing should be: "Nothing is ever constant and change will always be the order of the day."

4. "India’s population is among the youngest in the world."
This sentence is grammatically correct. The possessive form "India's population" is used properly, and "among" is correctly used to compare India's population within a larger group (the world).

Therefore, the sentence with the error is the third option.