**Solution:**
To solve this question, we need to analyze the given options and find a pattern or relationship between them. Let's analyze each option one by one:
a) 72
b) 78
c) 108
d) 90
From the given options, we need to find the number that can be placed at the question mark (?).
Now, let's analyze the given options:
Option a) 72: The number 72 is divisible by 9, as the sum of its digits (7 + 2 = 9) is also divisible by 9. This property makes it a possible candidate for the answer.
Option b) 78: The number 78 is not divisible by 9, as the sum of its digits (7 + 8 = 15) is not divisible by 9. Therefore, it is unlikely to be the answer.
Option c) 108: The number 108 is divisible by 9, as the sum of its digits (1 + 0 + 8 = 9) is also divisible by 9. This property makes it a possible candidate for the answer.
Option d) 90: The number 90 is not divisible by 9, as the sum of its digits (9 + 0 = 9) is divisible by 9. Therefore, it is unlikely to be the answer.
**Pattern:**
From the given options, we can observe that the numbers that are divisible by 9 (options a and c) have the sum of their digits divisible by 9 as well. On the other hand, the numbers that are not divisible by 9 (options b and d) have the sum of their digits not divisible by 9.
**Conclusion:**
Based on the pattern observed, we can conclude that the correct answer is option 'a' (72) because it satisfies the given pattern. The sum of the digits (7 + 2 = 9) is divisible by 9, which makes it a possible candidate for the answer.