Hi, Hussein Hassan,
The correct option is (d), but the capital in "However" is a typo:
They are only part of the application, however.
This "however" links the sentence with the one before, the idea being: SAT scores are extremely important, but they are only part of the application.
"teacher recommendations" is fine, because "teacher" is used generically. If the genitive case were used, this would be preferred: teachers' recommendations.