Read the following sentences and choose the ones belonging to the domain of complex sentences.
A. That Shankar is clever is certain.
B. I do not know whether the principal is in the office.
C. Rennie is not only kind but also devoted.
D. Can you tell me the reason why Mukti never smiles ?
Choose the correct answer from the options given below :

A complex sentence has a main clause that can stand alone and a subordinate (dependent) clause that cannot stand alone and is connected to the main clause, often introduced by subordinating conjunctions (e.g., "that," "whether," "why") or relative pronouns.

Option A: The clause "That Shankar is clever" is a noun clause acting as the subject of the sentence (dependent clause), and "is certain" is the main clause (independent) . This is a complex sentence because it has a dependent noun clause and an independent clause.

Option B: This is a complex sentence. "I do not know" is the main clause (independent clause), and "whether the principal is in the office" is a noun clause (which is dependent) acting as the object of the verb "know." 

Option C: This is a single clause with a compound predicate ("is not only kind but also devoted"). The phrase "not only kind but also devoted" is coordinated by the correlative conjunctions "not only...but also." This is not a complex sentence.

Option D: The main clause is "Can you tell me the reason," (independent) and "why Mukti never smiles" is a relative clause modifying "the reason." This is a complex sentence because it has an independent clause and a dependent relative clause.

Therefore, the sentences that are complex are A, B, and D.