Which of the following is the correct contraction for "it is"?

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The contraction for "it is" is represented correctly by the choice "it's." This contraction is formed by combining the pronoun "it" with the verb "is," which is often indicated by the use of an apostrophe. The apostrophe replaces the letter 'i' in "is," resulting in "it's." This usage helps to clarify the meaning in sentences where "it is" is intended to be conveyed more succinctly.

In contrast, "its" is a possessive form, meaning it shows ownership or belonging, which would not fit in this context. "Its'" is not a recognized form in standard English, as possessive nouns typically do not take an apostrophe when they are plural or singular possessive. Finally, "ithis" does not represent a correct contraction, as it resembles a misspelling rather than a legitimate form. Thus, "it's" stands out as the accurate contraction for "it is."