Just the fact that you say "yes" to one of those questions doesn't mean you will not be allowed in, your item will be confiscated, you will be fined, or anything bad at all will happen. It means the person you hand your card to will ask you some questions.
You can mark yes, circle the word "soil" and then write "beach sand" on the card. For example entering the US last week I marked yes to item a, food, and wrote "coffee beans, candy" on the card. Typically they ask you a few questions and if the item is allowed in, you're on your way. If it's not that clearcut they may ask you more questions, or look at the item, or search your luggage.
I do not know the official position on sand, but since many people have sandy shoes, clothes, towels and so on, I would suggest that if they cared they would have a question about being on a beach just as they have one about being on a farm (many other countries ask if you have been in a forest.) But as long as you declare it, you should be fine, at worst they will take it from you. That's the same as you leaving it behind, so I suggest you bring it and declare it.