Which of the following has an incorrect usage of the Possessive case?

Which of the following has an incorrect usage of the Possessive case? Correct Answer for conscience’s sake

In usage of the Possessive case, when the word has too many hissings sounds the letter ‘s’ is omitted after the apostrophe. The correct form should be ‘for conscience’ sake’.

Related Questions

Which of the following statements regarding the Possessive case is wrong?
Which one is possessive case?
The use of possessive case should not be confined to which of these?
pick the correctly written possessive case
Possessive case is also known as?