Explanation

Incorrect: They weren't artists they were detectives in disguise 

Correct: They weren't artists; they were detectives in disguise.

Explanation

Article “the” is used before a superlative degree of Adjective.

More useful is comparative degree while the most useful is the superlative degree.

Explanation

The following phrases end in “To” and are followed by “ing” form of a verb.

“Be used to, accustomed to, averse to, addicted to, in addition to, with a view to, devoted to, look forward to, object to, owing to, given to, taken to, prone to”.

Explanation

Sometimes, Prepositions are inserted where they are not required.

After having finished my work, I went home. (Incorrect Sentence)

Having finished my work, I went home. (Complete & Correct Sentence)

Explanation

Laugh is an intransitive verb.

Laugh at someone means کسی پر ہنسنا یا مذاق اڑانا 

Laugh is generally followed by at

We should not laugh at the poor means ہمیں غریبوں پر نہیں ہنسنا چایئے  

Explanation

Last month shows past tense.

Second form (threw) is used in simple sentence of Past Indefinite Tense,

Explanation

The verb 'Abide' is followed by the preposition 'By'

Abide by means to  accept or agree.

Explanation

"As far as I know, she is sick" is correct and commonly used. It means that based on the information you have, you believe she is sick.

"In my opinion, she is sick" is also correct. It expresses your personal belief or judgment.

So, the correct and more natural options are:

B) As far as I know, she is sick.

C) In my opinion, she is sick.

Explanation

Generally, people are forced to do something.

This is a sentence of Past Indefinite Tense (Passive Voice)

Explanation

incorrect: She will be very angry unless we do not reach there on time

correct: She will be very angry if we do not reach there on time

If is used to express a condition. 

We cannot use NOT after UNLESS.