<p style="">For each seminar, students have to register and pay before the registration date <strong><u>on which</u></strong> the seminar begins.</p><p style="">The correct answer is (C) <strong>on which</strong>.</p><p style="">In this sentence, we need a relative pronoun to introduce the relative clause "the seminar begins". The relative pronoun should refer back to the noun "registration date". The relative pronoun "which" is used to refer to a non-living thing or a date. In this case, "<strong>which</strong>" refers to the registration date.</p><p style="">The preposition "<strong>on</strong>" is there because, in English, we typically use the preposition "on" to refer to specific dates or days. For example: The meeting is on the 5th of July.</p><p style="">In the sentence you provided, "the registration date" is a specific date. So when we want to provide additional information about something happening on that date, we use "on which" as a relative pronoun + preposition combination.</p><p style="">Thus, "the registration date on which the seminar begins" effectively means "the registration date on (the date) the seminar begins." By using "<strong>on which</strong>," we maintain grammatical correctness while providing clarity regarding the event (the beginning of the seminar) occurring on that specific date (the registration date).</p><p style="">Therefore, the correct phrase is "on which".</p>