The core concept tested here is the understanding of the term 'engagement' in the context of social media. The correct answer is B, as 'engagement' in social media refers to the act of participating or becoming involved in something, such as interacting with content. Option A refers to a formal agreement to get married, which is not relevant to social media. Option C refers to a job or period of employment, which is also not relevant. Option D refers to a promise or agreement to do something, which is too broad and not specific to social media engagement.