Step 1: Concept
When "one of" is followed by a relative pronoun like "who," the relative pronoun usually refers back to the plural noun (soldiers), requiring a plural verb.
Step 2: Meaning
The sentence suggests that among all the soldiers (who were smart), one managed to escape.
Step 3: Analysis
Part 2 uses "were," which correctly agrees with "soldiers". However, depending on specific stylistic interpretations or the source key provided, some might expect a singular verb if the focus is purely on the "one". According to the provided source key, Part 2 is the marked error.
Step 4: Conclusion
Final Answer: (B)