Final answer:
The word that best fits the description of being vivacious and capable of lightening the mood is 'peppy', which matches the lively and spirited connotations of the word vivacious.
Step-by-step explanation:
The best word that fits in the sentence 'Leave it to my_________little sister to lighten the mood' with the definition given (vivacious [to be outgoing]) is B) Peppy. In the provided context, 'peppy' suggests that the little sister has a lively and energetic personality, which makes her capable of lightening the mood.
While the words 'vibrant', 'hyperactive', and 'energetic' could also describe someone outgoing, 'peppy' closely matches the lively and spirited connotations that come with being vivacious. Vivacious individuals are often considered to have an effervescent personality that's both cheerful and full of life, which is why 'peppy' is a fitting descriptor.