Answer: Option C. -> They were selling many types of fish at the market.
Answer: (c)Many types of fish were being sold by them at the market. (a) They have sold many types of fish at the market. (Tense has changed) (b) They are selling many types of fish at the market. (Tense has changed) (c) They were selling many types of fish at the market. (Correct) (d) They sell many types of fish at the market. (Tense has changed)

Answer: Option A. -> The old man was being laughed at by the students.
Answer: (a)

Answer: Option B. -> The police have arrested him and the court has sent him to jail.
Answer: (b)He has been arrested and sent to jail. (a) The police arrested him and sent to jail. (Incorrect change of tense) (b) The police have arrested him and the court has sent him to jail. (Correct) (c) The police has arrested him and the court has sent him to jail. (Incorrect word used) (d) None of these

Answer: Option C. -> The authorities shall prosecute trespassers.
Answer: (c)

Answer: Option C. -> Your kindness will never be forgotten.
Answer: (c)I shall never forget your kindness. (a) Your kindness shall not be forgotten.(Incorrect structure) (b) Your kindness is never forgotten. (Tense has changed) (c) Your kindness will never be forgotten. (Correct) (d) Your kindness should never be forgotten.(Incorrect word)

Answer: Option B. -> Shall we help Motin ?
Answer: (b)Will Motin be helped by us ? (a) Will Motin help us ? (meaning has changed) (b) Shall we help Motin ?(correct) (c) Shall Motin be helped? (doer is missing) (d) We shall helped Motin ? (incorrect structure)

Answer: Option A. -> He is told to get out of our house.
Answer: (a)

Answer: Option D. -> The dogs were chasing him
Answer: (d)

Answer: Option D. -> The results of the assembly elections are known to all of them.
Answer: (d)All of them know the results of the assembly elections. (a) The results of the assembly elections are being known to all of them. (Tense has changed) (b) The results of the assembly elections have been known to all of them. (Tense has changed) (c) The results of the assembly elections were known to all of them. (Tense has changed) (d) The results of the assembly elections are known to all of them. (Correct)

Answer: Option D. -> He was accused of various offences by his subordinates
Answer: (d)