The sentence is comparing Gopal to the other boys in the class. When using "as...as" to make a comparison of equality, we use the base form of the adjective (i.e., "smart") rather than the comparative form ("smarter").
Step 1: Understanding the comparison
The structure "as...as" is used for showing equality in English. For example:
\[
\text{No other boy is as smart as Gopal in the class.}
\]
Here, "smart" is the correct form because we are making an equal comparison.
Step 2: Conclusion
Thus, the correct answer is "smart."
Final Answer: smart.