Identify the most appropriate antonym of the given word.
Rapid

To find the most appropriate antonym of the given word, let us first understand its meaning:
Rapid means happening in a short time or at a great speed; fast or quick.

Now, let us analyze the meanings of the given options:
1. active: busy, moving about, or energetic (not an antonym of rapid).
2. smart: clean, tidy, or showing quick-witted intelligence (not an antonym of rapid).
3. lately: recently or in the immediate past (not an antonym of rapid).
4. slow: moving or operating at a low speed; not quick or fast.

Since "slow" is the direct opposite of "rapid", it is the most appropriate antonym.

Therefore, the correct answer is slow.