Let n =1 then,
if a = { 2 } , b = { 1 }
< a,b > = 2
|| b || = 1
Hence i is incorrect.
Let a = {-1} , b ={1}
< a,b > = -1
|| a || || b || = 1 x 1 = 1
Hence, iii and iv is incorrect, if iv is incorrect d and e both can't be right.
So, by elimination (a) is correct.