___ a shiny day today!

The correct choice is "it's," which is a contraction of "it is." This phrase conveys the message that the day is shiny, and the use of the contraction makes the sentence more conversational and fluid.

In English, contractions like "it's" are commonly used in informal writing and speech to express ideas more succinctly. For example, one might say, "It's a beautiful day," to indicate that the day is indeed beautiful.

Other options do not accurately fit the context. "Its" is a possessive pronoun, referring to something that belongs to "it," and would not make sense in this context. Phrases like "its is" and "it's is" are not grammatically correct in standard English usage, as they either misuse a possessive form or combine two contractions incorrectly. Thus, "it's" is the appropriate choice for expressing the idea of the shiny day.