What is the best way to index several foreign keys?

Question

Learning Oracle basics, I learned that foreign keys should be indexed, thanks to this question.

However if I have a table with several foreign keys, should I create 1 index for each foreign key or 1 index for the entire table ?

For example, I have the table A(a,b,c,d), b and c being foreign keys on tables B and C.

Is the best solution :

CREATE INDEX index_tab ON A(b,c);

Or should I go with :

CREATE INDEX index_A_1 ON A(b);
CREATE INDEX index_A_2 ON A(c);