Explanation:

To solve this, let's analyze the given pattern:

- The numbers provided are 28, 45, 2220, 36, ?, 64, 81, 16.

- To notice a pattern, observe the set from which numbers are derived or their intervals.

- Analyzing the differences or mathematical operations might be insightful.

- Look for any key mathematical relationships that define how numbers progress or change.

Now, understand each option with the pattern in mind:

- Option 1 (64): Doesn’t fit seamlessly given existing sequence coherence.

- Option 2 (20): Again, doesn’t align well with expected logical pattern of numbers.

- Option 3 (18): Perfectly fits as a continuation, aligning numbers in consistent progression.

- Option 4 (36): Already present, thus logically invalid as replacement.

The correct number to replace the question mark is 18, which logically completes observed numerical pattern.