a)    Because Arjuna was doubtful of believing Krishna’s words
b)    Because Arjuna asked this for demoniac people who don’t accept Krishna to be the Supreme Personality of Godhead
c)    Because Arjuna was himself not aware of the fact that Krishna is the Supreme Personality of Godhead
d)    All of the above 

The correct answer is
b)    Because Arjuna asked this for demoniac people who don’t accept Krishna to be the Supreme Personality of Godhead