Which of the following numbers will replace the question mark (?) in the given series?
29, 30, 26, 35, 19, ?

The correct answer is 44.

To find the missing number in the series, let us analyze the differences between consecutive terms:

First term to second term: 30 - 29 = +1 (which is +1²)
Second term to third term: 26 - 30 = -4 (which is -2²)
Third term to fourth term: 35 - 26 = +9 (which is +3²)
Fourth term to fifth term: 19 - 35 = -16 (which is -4²)

The pattern shows that the differences are alternating in sign (+, -, +, -, ...) and are the squares of consecutive positive integers (1², 2², 3², 4², ...).

Following this pattern, the next difference should be:

$+5^2=+25$

Adding this difference to the fifth term (19) gives the next number in the series:

$19+25=44$