Step 1: Concept
Two linear equations are sufficient for two variables only if they are independent.
Step 2: Analysis of Statement I
$x + 2y = 6$. One equation, two variables. Infinitely many solutions for $x$. Not sufficient.
Step 3: Analysis of Statement II
$3x + 6y = 18$. Divide by 3: $x + 2y = 6$. Same as Statement I. Not sufficient.
Step 4: Combined Analysis
Since Statement II is just a multiple of Statement I, they are "dependent" and represent the same line. There is still no unique value for $x$.
Step 5: Conclusion
Correct choice is (4).
Final Answer: (4)