Choice B is the correct answer. Let's analyze each system:

System I: $y = 3x + 1$ and $y = 3x - 2$

• Same slope (3), different y-intercepts
• Parallel lines → No solution

System II: $y = 3x + 1$ and $y = -3x + 1$

• Different slopes (3 and -3)
• Intersecting lines → Exactly one solution ✓

System III: $y = 3x + 1$ and $3y = 9x + 3$

• Divide second by 3: $y = 3x + 1$
• Same line → Infinitely many solutions

Conclusion: Only System II has exactly one solution.

💡 Strategic Tip: Different slopes guarantee exactly one solution (lines must intersect).

Choice A is incorrect because System I has no solution (parallel lines).

Choice C is incorrect because System III has infinitely many solutions.

Choice D is incorrect because System III has infinitely many solutions, not one.