KarunMendiratta has provided a nice solution that uses counting techniques.
Here's a solution that involves probability rules

First recognize that the 1st selected letter must be A, B, C, D, E or F, otherwise it will be impossible to have three consecutive letters (e.g., if we select G first, then the next two letters would need to be H and I, but the letters only go up to H).

So,....
P(the 3 selected letters are in order) = P(1st selected letter is A, B, C, D, E or F AND the 2nd letter is the letter than follows the 1st selected letter AND the 3rd letter is the letter than follows the 2nd selected letter)
= P(1st selected letter is A, B, C, D, E or F) x P(2nd letter is the letter than follows the 1st selected letter) x P(3rd letter is the letter than follows the 2nd selected letter)
= 6/8 x 1/7 x 1/6
= (6)(1)(1)/(8)(7)(6)
= (1)(1)/(8)(7)
= 1/56

Answer: C