Answer:
Politicians have discovered that social media sites work <em><u>beautifully</u></em> for campaigning.
Explanation:
Taking the two words given in the option, let's differentiate them.
First, beautiful is an adjective and can be used to describe a noun. On the other hand, beautifully is an adverb that is used to describe a verb.
Now, considering the given sentence, the word near the 'blank' is "work' which is a verb. And so, using "beautiful" is invalid, making "beautifully" the only other option to be used.
And, if we put "beautifully" in the black, it completes the sentence.
Thus, the <u>correct answer is option b.</u>