The term serious refers to acting or speaking seriously and earnestly, as opposed to joking.
Option A, merry, relates to being cheery and pleasant.
Option B, thoughtful, relates to giving careful thought and attention.

Option C, smug, denotes undue pleasure in oneself or one's accomplishments.

Option D, insincere, relates to failing to show genuine emotions. For example, John is so insincere you can not guess if he is telling you the truth.

Thus, the meaning of the word 'insincere' is the inverse of the meaning of the word serious.'