Answer:
If the restaurant is not very crowded, you can place your child on another chair about 3 to 4 feet away from you.
Explanation:
The given sentence is from a list of advice you can use to teach your children good behavior in public places. The word that is missing is crowded.
The only two options that make sense are crowded and full. A restaurant can't be invisible, and this advice wouldn't make much sense if the restaurant wasn't very empty. Besides that, this combination of words sounds awkward.
Full isn't the correct option because a restaurant is either full or not. It can't be not very full or very full. But, it can be more or less crowded as there can be more or fewer people in there. This is why option B is the correct one.
