Consumer behavior is $\_\_\_\_.$

a$.$ the activity, set of institutions, and processes for creating, communicating, delivering, and exchanging offerings with value for individuals, groups, and society

b$.$ the process by which a consumer comes to own or experience an offering

c$.$ the totality of consumers' decisions with respect to the acquisition, consumption, and disposition of goods, services, time, places, and ideas by human decision$-$making units

d$.$ the study of how people buy products