One segment of the sentence contains an error.
I am happy / to know that / her brother speaks German good.

Correct option is A

Option (a) contains an error.
Explanation: The word “good” is an adjective, but here an adverb is needed to describe the verb "speaks". The correct form is “well” (adverb of manner).
Correct sentence: Her brother speaks German well.
Grammatical Rule Used: Use adverbs, not adjectives, to describe how an action is performed.
�� Information Booster:
· Adjective: She is a good singer.
· Adverb: She sings well.