8 : 72 : : 9 : _____

The correct option is C: 90.

Detailed Solution:

1. The relationship between 8 and 72 is determined by multiplication: 
   8 × 9 = 72.
2. Similarly, for 9, the same multiplier (9) is used: 
   9 × 10 = 90.
3. Thus, the missing number is 90.

Concept Behind the Problem:

This problem is based on proportional relationships and multiplication:

• The first pair (8 : 72) is connected by multiplying the first number by a fixed value (8 × 9).
• The same logic is applied to the second pair (9 : _____) to find the missing number (9 × 10).

This type of problem tests your ability to identify patterns and relationships between numbers.