Explanation

The pattern appears to be the number of letters in each word:

DRIVER has 6 letters, but the value is 7 ( possibly due to a additional constant or a different calculation)

PARLIAMENTARIAN has 15 letters, but the value is 16 (again, possibly due to a additional constant or a different calculation)

If we apply the same pattern to SECRETARY, which has 9 letters, we get:

SECRETARY = 10 (using the same offset as before)

So, the correct answer is: 10