Select the most appropriate option that can substitute the highlighted segment in the given sentence.
The door which connecting the two offices is kept locked.

Correct option is A

The correct substitution of the highlighted words is (a) connects.
Explanation: The subject The door is singular and needs a finite verb in the simple present to state a general fact: The door connects the two offices.
The highlighted segment which connecting is ungrammatical: after which (a relative pronoun), we need a finite verb ( which connects), not a participle ( connecting).
Since the options don’t offer which connects, we rewrite the sentence without the relative pronoun and use the simple present verb connects, which is concise and correct.
Grammatical rule used: Use simple present for general truths/functions; in relative clauses, the relative pronoun is followed by a finite verb ( which + V-s/V2/etc.). A reduced relative clause may use the -ing form without which:
The door connecting the two offices…
Example: The bridge connects the island to the mainland.
Information booster: Reduced relatives: The students sitting in the front row… (= who are sitting…). But if you keep which/who, use a finite verb: which sit / sits , not which sitting.