Answer: Loudly/well
Explanation: On one side the words LOUD and GOOD are adjectives. Adjectives are words used to describe nouns. On the other side the words LOUDLY and WELL are adverbs. Adverbs are used to describe verbs. In the example given the first word describes the verb "talking" and the second word describes the verb "concentrate" this is why two adverbs are needed, loudly and well.
