The group was accused of biased participation in the discussion because their entire attention was centred around their own proposal.

Correct option is A

· Centred on (a): Correct, "centred on" is the appropriate phrase for focusing attention or efforts on a particular point or topic.
· Centred in (b): Incorrect, "in" suggests a physical or metaphorical enclosure, not suitable for discussing focus.
· Centred over (c): Incorrect, "over" generally denotes a position above or covering, which doesn't align with the context of focusing attention.
· No improvement (d): Incorrect as "centred around" suggests a vague, general vicinity rather than a specific focus point.
Grammar Rule: The correct preposition after "centred" is crucial for conveying the precise relationship between the subject's attention and its object.