FIRST(S)
Let the set be k={}
FIRST(S) = see first what A is deriving
A -> BD
now see what B is deriving
case 1 : B->b
so A->bD
so k ={b}
case 2 : B-> ɛ
so A->D
now see what D is deriving
case 2.1 : D->d
so A->d
so k ={b,d}
case 2.2 : D-> ɛ
so A-> ɛ
putting A-> ɛ in S
so S->a
so k = {b,d,a}
so FIRST(S) = {b,d,a}