Explanation: In English grammar, prepositions are used to indicate the relationship between a noun or pronoun and other parts of the sentence. When it comes to time expressions, prepositions play a crucial role in specifying the exact time, date, or period being referred to.
For days of the week, the preposition "on" is specifically used to denote a particular day. For example, "They eat out on Sundays." This usage is consistent across various contexts and is a fundamental rule in English grammar.
Let's break down the usage of prepositions with time expressions:
- "On" is used for specific days of the week, dates, and holidays. For example, "on Monday," "on 15th June," "on Christmas Day."
- "In" is used for larger time periods such as months, years, seasons, and parts of the day. For example, "in June," "in 2023," "in summer," "in the morning."
- "At" is used for specific times or points in time. For example, "at 3 PM," "at midnight," "at the moment."
In the given sentence, "They eat out ... Sundays," the correct preposition to use is "on" because it refers to a specific day of the week. Using "in" or "at" would be incorrect as they do not align with the rules for specifying a particular day.
Understanding these preposition rules is essential for constructing grammatically correct sentences and for effective communication in English. It is a common topic in various competitive examinations in India, including UPSC, SSC, Bank PO, and CLAT, where questions on grammar and language usage are frequently asked.
To summarize, the correct answer is "on" because it is the appropriate preposition to use when referring to a specific day of the week. This rule is fundamental in English grammar and is crucial for both written and spoken communication.