Final answer:
The term 'interactions' best applies to 'C. two strangers striking up a conversation about things in common,' as it involves active engagement and communication, which is a key aspect of social interaction.
Step-by-step explanation:
The term interactions refers to situations where individuals are actively engaging with one another through direct communication or influence. Considering the provided options, C. two strangers striking up a conversation about things in common best applies to interactions. This scenario involves direct communication and exchange of information between the individuals, which is a form of social interaction.
On the other hand, a child watching others, someone reminiscing, and someone waiting alone do not encompass active engagement between multiple parties, thus they do not best embody the concept of interactions as described in the field of social studies.