The sentence that correctly shows how the apostrophe is used to form a contraction is:
"fred noted its a poor time to plant spring flowers until after the last frost."
However, it's worth noting that this sentence contains a grammatical error. The correct contraction should be "it's" (short for "it is"). So the corrected version should read:
"Fred noted it's a poor time to plant spring flowers until after the last frost."
In this context, "it's" is the contraction representing "it is."