Which of the following is an example of the contraction often used with the genitive case?

The correct answer is an example of the contraction related to the genitive case because "John's" indicates possession. In English, the genitive case often shows ownership or association, typically formed by adding an apostrophe and the letter "s" to a noun. In this instance, "John's" signifies that something belongs to John, demonstrating the possessive form. This construction is an essential aspect of expressing possession in English, which is a key feature of the genitive case, making it the right choice for this question.

The other options do not represent the genitive case; they are contractions of auxiliary verbs or other phrases and do not imply possession. "They will" combines a subject pronoun with a modal verb, "It has" conjoins a subject pronoun with the present tense of the verb "to have," and "She is" merges a subject pronoun with a form of the verb "to be." Therefore, they do not exemplify the concept of possession central to the genitive case.