With the help of the given options, fill in the blank in the following sentence.
I knocked ________ the door but no one answered.

Correct option is C

The correct option to fill in the blank is (c) at.
Detailed Explanation (English):
• The correct collocation is “knock at the door” when we mean to strike the door to get attention.
• Therefore, “knocked at the door” is grammatically correct and commonly used.
Correct sentence: I knocked at the door but no one answered.
Grammatical Rule Used (English):
• Fixed collocation: knock at + door/window (for attention).
Grammatical Rule Used (Hindi):
• “door” पर ध्यान आकर्षित करने के लिए सही प्रयोग “knock at the door” होता है।
• यह एक fixed usage/collocation है।
Meanings of the other options:
• (a) Into: shows movement/collision to the inside (के अंदर/से टकराकर)
• (b) In: shows inside/within (के अंदर)
• (d) Of: shows belonging/relation (का/के)