Topic: Conjunction,

Best Answer Explained in Below-:

The sentence pairs "You are celebrating the birthday" and "Your friend is dead serious" present two contrasting situations. To effectively link these disparate ideas, we need a conjunction that emphasizes their difference. The word "whereas" fits this purpose perfectly.

"Whereas" is used to highlight a stark contrast between two ideas. In this case, while one person is joyfully celebrating a birthday, the other person is in a very serious mood, indicating a notable divergence in emotional states or activities. The use of "whereas" allows the reader to understand that these two scenarios are not only different but also occur simultaneously, enhancing the depth of the comparison.

In contrast, the other options do not convey this contrast effectively. "Since" suggests causation, "whence" refers to a source, and "as" can imply time or reason, none of which suitably express the opposing natures of the two situations. Therefore, "whereas" is the appropriate choice for clearly illustrating the divergent circumstances in the given context.