There's a common misconception that it's wrong for an answer choice to "change the meaning." There's no original meaning you need to preserve. Instead, think of it this way: there are five sentences. You want the one that's clearest and most logical.

Take another look at (C):


This option is saying that some places have no tides at all, while others have tides that are more than 30 feet tall. That seems perfectly fine. Surfer A is sitting on a calm, placid sea, bummed that he can't catch a wave. Surfer B is catching massive waves, over 30 feet tall. Maybe they're 31 feet tall; maybe they're 46 feet tall. This seems perfectly logical and clear.

You noted correctly that, in (A), the phrase, "at least 30 feet and more," is redundant. Worse, it's confusing. If we know that "at least 30 feet" captures any value that's 30 or greater, "the more" seems like it might refer to something other than height. But nothing else is specified, so the reader is left scratching her head, trying to puzzle out the meaning. At least 30 feet and more... dangerous? At least 30 feet and more... full of sharks? Who knows?

So rather than worry about whether (C) changes the meaning, ask yourself whether (C) is clearer. Because it is, it's a better answer choice.

I hope that helps!
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