The mistake is in option B. It should be "Everyone at the party was happy and laughing a lot." The verb "to be" is used with singular subject pronouns (such as everyone), and "laughing" should be in the continuous tense to match the present participle form of "happy."

The mistake is in option B. It should be "The girl told me that she was cleaning the floor now." In reported speech, the verb "to say" is usually followed by "that," and the past continuous tense is used to describe an action that was happening at a specific time.

The mistake is in option C. It should be "My mother is interested in reading books, especially novels." We use the adjective "interested" to describe how someone feels about something, not "interesting," which is used to describe the thing itself.

1. The mistake is in option B. It should be "It is such a difficult question that no one can answer." "So" is not used to emphasize adjectives in this context; we use "such" instead.

The mistake is in option A. It should be "I have been learning English since I was 10 years old." The present perfect continuous tense is used to describe an action that started in the past and continues up to the present moment.