Answer:
d.informal
Step-by-step explanation:
In group relationship, it is classified as a result of the type of relationship which the individual engaged in. For example, the type of relationship between a grandson and his grandfather would be classified differently from a relationship between two flatmates that lives together despite the fact that they are not sharing same father or mother.
In the case of Susan and Mary, their relationship would be classifed as an informal group relationship. This is due to the fact that they are not family members but neighbours who happens to have known each other over the past few years.
Due to the reason they share personal details about each other's farmily among each other means that their relationship is under informal group relationship.