In diborane, 2 electrons are shared between one hydrogen atom and two boron atoms. therefore answer should be diborane

Diborane (B2H6) is the compound that has a 3-centre bond.

Explanation:
Diborane is a chemical compound composed of boron and hydrogen atoms. It is a highly reactive and flammable gas. The structure of diborane consists of two boron atoms bonded to each other, with six hydrogen atoms attached to the boron atoms.

In diborane, the boron-boron bond involves a 3-centre, 2-electron bond. This type of bond is also known as a banana bond. It is formed by the overlap of two atomic orbitals from each boron atom with a hydrogen atom. This bonding arrangement allows for the sharing of electrons between the boron atoms, resulting in a stable structure.

The presence of a 3-centre bond in diborane can be understood by considering the electronic configuration of boron. Boron has three valence electrons, and in diborane, each boron atom contributes one electron to the 3-centre bond. The remaining two valence electrons on each boron atom are involved in a normal covalent bond with two hydrogen atoms.

The 3-centre bond in diborane is responsible for its unique properties. It contributes to the stability of the compound and makes it an effective reducing agent. The presence of the 3-centre bond also influences the geometry of diborane, which is characterized by a bridged structure.

In contrast, carbon dioxide (CO2), boron trifluoride (BF3), and ammonia (NH3) do not have 3-centre bonds. Carbon dioxide has a linear structure with double bonds between the carbon and oxygen atoms. Boron trifluoride has a trigonal planar structure with three sigma bonds between boron and fluorine atoms. Ammonia has a pyramidal structure with a lone pair of electrons on the nitrogen atom and three sigma bonds.

Therefore, the correct answer is option A) Diborane.