Final answer:
The best choice for describing someone who is filled with happiness is 'overjoyed'. Other synonyms like 'glad', 'pleased', and 'pleasant' also express happiness but to a lesser extent.
Step-by-step explanation:
The best choice for describing someone who is filled with happiness is overjoyed. When someone is overjoyed, they are experiencing an intense and overwhelming feeling of happiness. This word conveys a high level of excitement and enthusiasm.
Other synonyms such as glad, pleased, and pleasant also express happiness to some degree, but they may not capture the same level of extreme joy as the word overjoyed.
Learn more about Synonyms for happiness