Which of the following numbers will replace the question mark (?) in the given series?
6, 10, 15, 21, ?, 36, 45

The correct option is 28.

To find the number that replaces the question mark (?) in the given series, we need to analyze the pattern of differences between consecutive numbers in the sequence:
6, 10, 15, 21, ?, 36, 45

Let's calculate the difference between each consecutive term:
From 6 to 10: $10-6=4$
From 10 to 15: $15-10=5$
From 15 to 21: $21-15=6$

We can observe that the differences between consecutive terms are increasing by 1 each time (+4, +5, +6, ...). If this pattern continues, the next difference should be +7.

Adding 7 to the last known term (21):
$21+7=28$

Let's verify if this pattern holds true for the remaining terms of the series:
If the missing term is 28, the next difference should be +8:
$28+8=36$ (which matches the next term in the series)
The difference after that should be +9:
$36+9=45$ (which matches the final term in the series)

Since the pattern of increasing consecutive integers (+4, +5, +6, +7, +8, +9) perfectly generates all terms, the number that replaces the question mark (?) is indeed 28.