The correct sentence is:

Rajan has three brothers and three sisters.

Explanation: The verb "has" is the correct form to use with the pronoun "Rajan." The original sentence, "Rajan is having," is not grammatically incorrect but sounds less natural in this context. Using "has" makes the sentence sound more standard and common in English.

The correct sentence is:

Our office is on the 2nd floor of the skyscraper.

Explanation: When referring to the location of a place within a building, we use "on" instead of "in." Therefore, the correct preposition in this context is "on the 2nd floor."

The correct sentence should be:

The hotels were fully booked for the festival in Banaras.

Explanation: The phrase "fully booked" means that all the available rooms in the hotels were reserved, which is the appropriate expression for indicating that there are no more rooms available for booking.

The correct sentence should be:

A team of biologists has discovered four new species of horned frogs in the North Eastern region of India.

Explanation: The subject "team of biologists" is singular, so the verb should be singular as well. "Has discovered" is the correct verb form to use in this case.

The correct sentence should be:

Hardly had we stepped out of the building when we got drenched.

Explanation: The phrase "then we get drenched" should be replaced with "when we got drenched" to maintain the correct verb tense. "When" is the appropriate subordinating conjunction to use in this context.

The correct sentence should be:

I couldn't sleep because I was very tired.

Explanation: The phrase "in case of being very tired" is awkward and not commonly used in English. Instead, you can use the phrase "because I was very tired" to clearly express the reason for not being able to sleep.

The underlined segment in the given sentence does not need substitution. It is already grammatically correct:

The Ghats in Benaras were very crowded but we were able to watch the Ganga aarti clearly.

The correct sentence should be:

He couldn’t believe it at first, so the more he thought about it, the more he decided it must be true.

Explanation: The phrase "the most he thought about it" should be replaced with "the more he thought about it" for the sentence to be grammatically correct. The phrase "the more" is used to indicate an increase in intensity or degree, which is appropriate in this context.