Question

...... his surprise, the door was open. 1.to 2. by 3. with​

Answer 1

To his surprise, the door was open.

Option (a) is correct.

Explanation:

Preposition:

• In English grammar prepositions are used to connect words with each other in a sentence.
• The prepositions usually precede a noun or pronoun.
• Prepositions express relation of words and sentences with each other in a clause.
• For example  on, at, under and by are prepositions.

Complete sentence:

To his surprise, the door was open.