A dyad is the smallest possible social group, consisting of two people. Sociologically, dyads are considered the most intense and fragile type of group because the relationship depends entirely on the participation of both individuals; if one person leaves, the group ceases to exist.

In sociology, a social group consists of two or more people who interact with one another, share similar characteristics, and have a sense of unity. The members of a hospital's business office interact regularly and share common goals, making them a primary or secondary group. In contrast, categories like age groups or residents of a city are social categories or aggregates, not groups, because they lack consistent interaction.

Social groups provide essential emotional and practical support that can significantly improve health outcomes. Studies have shown that participation in support groups can reduce stress, improve coping mechanisms, and even extend life expectancy for individuals facing severe medical conditions, such as cancer, by providing a sense of community and shared experience.

The family is fundamentally a social group, as it consists of individuals interacting with one another based on shared roles and expectations. While it is also a social institution that performs vital functions for society, the question asks for its classification as a group. Note: Some academic contexts might argue for 'Both A and B', but based on the provided key, 'Social' is the intended answer.

Intergroup relationships refer to the interactions, dynamics, and social connections between different social groups. These relationships are central to understanding social stratification, conflict, and cooperation. Sociologists study these interactions to determine how group identity, power dynamics, and social boundaries influence the way different segments of society relate to one another in various social contexts.

Sociologists, most notably Charles Horton Cooley, distinguish between primary groups, which are characterized by intimate, face-to-face association and cooperation, and secondary groups, which are larger, more impersonal, and goal-oriented. These two categories form the fundamental basis for analyzing social interaction.

Social loafing is the tendency for individuals to exert less effort when working collectively than when working individually. This occurs because individual contributions are often less identifiable in a group, leading to a diffusion of responsibility where members rely on others to complete the task, thereby reducing their own personal output.

An aggregate consists of people who are in the same place at the same time but share little to no social interaction or common identity. Unlike a social group, which involves sustained interaction and shared goals, people waiting for a bus are merely physically proximate.

Expressive leaders are those who prioritize the emotional health and morale of group members. Unlike instrumental leaders, who focus on completing specific tasks and achieving goals, expressive leaders work to maintain group harmony and provide the necessary emotional support to ensure that individuals feel valued and connected within the social unit.

Leadership in groups often involves a mix of styles. Instrumental leadership focuses on achieving specific goals and tasks, while expressive leadership focuses on group cohesion and emotional well-being. Effective leaders often integrate these approaches, and the concept of 'ideal leadership' is frequently discussed in theoretical frameworks as a balance of these dimensions.