A sentence is given with an underlined part. Choose the alternative that will improve the sentence. In case no improvement is needed, select No improvement: When in doubt check it up with a good dictionary.

Introduction / Context:
This sentence improvement question focuses on the correct use of verbs and prepositions in a common advisory expression. The sentence gives advice about what to do when one is uncertain about a word or usage. The phrase check it up with a good dictionary is slightly unnatural in standard English, and the task is to pick the option that sounds natural and grammatically correct.

Given Data / Assumptions:

• Original sentence: When in doubt check it up with a good dictionary.• Underlined part: check it up with.• Options: check it up in, check it in, check it with, No improvement.• The intention is to advise consulting a dictionary to confirm meaning or spelling.

Concept / Approach:
In modern English, the natural collocation is check something in a dictionary or look something up in a dictionary. We either use check with a person for confirmation, or look up something in a reference book. The given sentence combines check it up with, mixing these patterns in an incorrect way. The best correction is to use check it in a good dictionary, which follows a standard and widely used structure.

Step-by-Step Solution:
Step 1: Identify what needs to happen when there is doubt, namely, consulting a dictionary for verification.Step 2: Recall the normal phrasing in English: check it in a dictionary or look it up in a dictionary.Step 3: Evaluate option B check it in, which gives When in doubt check it in a good dictionary, a fully natural sentence.Step 4: Examine option A check it up in, which tries to mirror look it up in but still awkward because check up is usually used with on, as in check up on someone.Step 5: Consider option C check it with, which would be more suitable when confirming with a teacher or friend rather than a book.Step 6: Option D No improvement cannot be chosen because the original mix of up with is not idiomatic for consulting a dictionary.

Verification / Alternative check:
Read each possible sentence aloud. When in doubt check it in a good dictionary sounds exactly like the advice given by teachers and grammar books. When in doubt check it up in a good dictionary feels clumsy, given that the more common phrasal verb is look up. When in doubt check it with a good dictionary sounds slightly off because with typically introduces a person or tool used for comparison, not a standard reference. Therefore, check it in is the best fitting choice for this sentence.

Why Other Options Are Wrong:
check it up in incorrectly mixes check and look up, and the phrasal verb check up ordinarily pairs with on. check it with suggests using the dictionary almost like a person to confer with, which is less natural in this context. No improvement keeps the awkward and unidiomatic phrase check it up with. None of these alternatives match the smooth and widely accepted expression check it in a dictionary.

Common Pitfalls:
One common error is to overuse phrasal verbs like look up and check up, and then attach them to incorrect prepositions. Another is to consider with correct simply because the dictionary is being used as a tool. A simple way to avoid such confusion is to recall actual textbook lines or instructions from teachers, which almost always say check it in the dictionary or consult a dictionary when uncertain.

Final Answer:
The sentence is best improved as When in doubt check it in a good dictionary.