A B-flat major triad consists of the notes B-flat, D, and F.
If we consider the options you provided (C, D, E, or F#), the only note that is part of the B-flat major scale is D. Therefore, from your options, D is the only note that is related to B-flat major.
C and E are not part of the B-flat major scale, and while F is part of the B-flat major triad, F# is not.