In which of the following sentences is the word its used correctly? a. the monster reared its head at the tourists. b. when trying to catch fish, its important to be patiet. c. its easy to learn to play the guitar, if you're willing to practice. d. the problem with the lawn mower is that its old.

The sentence which uses punctuation correctly is The monster Reared its head at the tourists. This is the only sentence that does not require the apostrophe, therefore it s the correct sentence. An easy way to distinguish whether or not a word needs an apostrophe or not is to see if it is is provided in my following example
It is a cloud day.
It's a cloudy day.
Are these two sentences equal?? Yup! And that is how you know. :)
Hope this helped you if you need anymore hep or further explanation just let me know!! :)