6  :  18  :  :  4  :  _

Step 1: Analyze the relationship between the first pair of numbers (6 and 18).

• Observe that 6 multiplied by 3 equals 18.
• Alternatively, 6 squared (6²) equals 36, and 36 divided by 2 equals 18.

Step 2: Apply the same pattern to the second pair, starting with 4.

• Multiplying 4 by 3 gives 12. However, 12 is not among the provided options.
• Using the second method: 4 squared (4²) equals 16, and 16 divided by 2 equals 8.

The number that completes the analogy is 8. Note: This problem involves identifying a consistent mathematical relationship between pairs of numbers. Recognizing such patterns is essential for solving analogies and number series questions effectively.