A number of the workers is working for that company.
Answer: A
Correction: "A number of" is treated as plural, so it should be:
"A number of the workers are working for that company."

Minh's parents are proud about his excellent final tests.
Answer: B
Correction: The correct preposition is "of," not "about."
"Minh's parents are proud of his excellent final tests."

Keep silent! Our grandmother is sleep in her room.
Answer: C
Correction: It should be "sleeping" instead of "sleep."
"Keep silent! Our grandmother is sleeping in her room."

Many foreigners have been attracted by the beautiful of Sa Pa.
Answer: B
Correction: "Beautiful" should be "beauty."
"Many foreigners have been attracted by the beauty of Sa Pa."

I haven't visited Mr. Ba when he moved to his new house.
Answer: A
Correction: It should be "since" instead of "when," as you're referring to a point in time from the past until now.
"I haven't visited Mr. Ba since he moved to his new house."

Would you mind help me with my Math exercise because it is difficult?
Answer: B
Correction: It should be "helping" instead of "help."
"Would you mind helping me with my Math exercise because it is difficult?"

Are they interested on writing English?
Answer: C
Correction: The correct preposition is "in," not "on."
"Are they interested in writing English?"

A number of reporters was at the fan meeting yesterday.
Answer: A
Correction: "A number of" is plural, so it should be "were" instead of "was."
"A number of reporters were at the fan meeting yesterday."

Ben is strongly enough to carry that bookshelf to the third floor.
Answer: B
Correction: It should be "strong enough" instead of "strongly enough."
"Ben is strong enough to carry that bookshelf to the third floor."

Mai got wet because of she forgot her umbrella.
Answer: C
Correction: It should be "because" instead of "because of."
"Mai got wet because she forgot her umbrella."

Rita drank too much and she drove dangerous.
Answer: C
Correction: "Dangerous" should be "dangerously."
"Rita drank too much and she drove dangerously."

Tom has a bike, so he always walks to school.
Answer: D
Correction: "Always" should be replaced with "never" because the meaning suggests he doesn't need to walk if he has a bike.
"Tom has a bike, so he never walks to school."

They have learned English in 5 years.
Answer: D
Correction: The correct preposition is "for" instead of "in" when referring to a duration of time.
"They have learned English for 5 years." (Alternatively, "They have been learning English for 5 years.")

Mrs. Smith together with her sons are away for holiday.
Answer: C
Correction: "Together with" is a singular subject, so "are" should be "is."
"Mrs. Smith together with her sons is away for holiday."

I'm looking forward to have a holiday.
Answer: B
Correction: It should be "having" instead of "have."
"I'm looking forward to having a holiday."