To solve this question, the user needs to know the correct subject-verb agreement. The sentence has a subject "Two-fifths of the troops" that requires a verb that agrees with it in number.
Options A, C, and D are incorrect because they use the singular verb form "was" or "has", which does not agree in number with the plural subject "Two-fifths of the troops."
Option B is correct because it uses the plural verb form "were," which agrees in number with the plural subject "Two-fifths of the troops."