Explanation:

Introduction:
The bond order is a measure of the stability of a covalent bond. It gives an indication of the strength and length of the bond. The bond order is calculated by subtracting the number of electrons in antibonding molecular orbitals from the number of electrons in bonding molecular orbitals and dividing the result by 2.

Bond order in He2 molecule:
The He2 molecule is formed by the combination of two helium atoms. Each helium atom has two electrons, and when they combine, they form a molecular orbital diagram with two bonding molecular orbitals and two antibonding molecular orbitals. The bonding molecular orbitals are lower in energy, while the antibonding molecular orbitals are higher in energy.

The bonding molecular orbitals are filled by the two electrons from each helium atom, resulting in a bond order of 1. The bond order is calculated as (number of electrons in bonding orbitals - number of electrons in antibonding orbitals)/2 = (2 - 0)/2 = 1.

Excitation of an electron:
When one of the electrons in the He2 molecule is excited to a higher energy level, it moves from a bonding molecular orbital to an antibonding molecular orbital. This means that the number of electrons in the bonding orbitals decreases by 1, while the number of electrons in the antibonding orbitals increases by 1.

Effect on bond order:
To calculate the new bond order, we need to subtract the number of electrons in the antibonding orbitals from the number of electrons in the bonding orbitals and divide the result by 2.

Before excitation:
Number of electrons in bonding orbitals = 2
Number of electrons in antibonding orbitals = 0

Bond order = (2 - 0)/2 = 1

After excitation:
Number of electrons in bonding orbitals = 1
Number of electrons in antibonding orbitals = 1

Bond order = (1 - 1)/2 = 0

Conclusion:
Therefore, when one of the electrons in the He2 molecule is taken to the next excited state, the bond order decreases by 1 unit from 1 to 0. The correct answer is option 'B', not option 'A'.