The right answer is 8 (2^3)
to solve these kind of question, you should use this equation:
solution = number of kinds of elements (aminoacids) ^ the lenght of the sequence (polypeptide).
If they are not numerous you can write those possibilities: (K for lysine and H for histidine).
1. KKK
2. KKH
3. KHK
4. KHH
5. HKK
6. HKH
7. HHK
8. HHH