Three Centre two electron bond is present in:(A).N{{H}_{3}} (B).{{B}_{2}}{{H}_{6}} (C).BC{{l}_{3}} (D).AlC{{l}_{3}}

The concept of a three-center two-electron bond is commonly associated with compounds in which three atoms share two electrons in bonding. Let's analyze each option:

A. NH₃ (Ammonia): NH₃ does not possess a three-center two-electron bond. It forms a conventional covalent bond with each hydrogen atom.

B. B₂H₆ (Diborane): Diborane contains a three-center two-electron bond, making it a valid candidate. The bridging hydrogen atoms contribute to this bond.

C. BCl₃ (Boron trichloride): BCl₃ does not contain a three-center two-electron bond. Each chlorine atom forms a conventional covalent bond with boron.

D. AlCl₃ (Aluminum trichloride): AlCl₃ does not contain a three-center two-electron bond. Each chlorine atom forms a conventional covalent bond with aluminum.

Therefore, the correct answer is option (B) B₂H₆ (Diborane), as it contains a three-center two-electron bond.