This is a combination problem. For the first letter, there are 26 possible letters in the alphabet to use. For the second, we cannot use the letter used in the first, so there are only 25 possible letters. Repeat this for the remaining 5 letters.
1st - 26
2nd - 25
3rd - 24
4th - 23
5th - 22
6th - 21
7th - 20
To find the number of possible passwords, simply multiply all these numbers together. 26*25*24*23*22*21*20 = 3,315,312,000. Therefore B is the correct answer.
