1) Average = 1.5
This doesn't give any information about individual prices of books.
All we know is
(X + y + z) = 4.5
There are many combination which satisfy this.
2) say x = 1.5
we need median of 1.5 , y ,x
combining 1 and 2
average is 1.5 and one of the book 1.5
That means among rest of the 2 books one book cost s more than 1.5 and one cost s less than 1.5
Both costs 1.5 each
In both the cases 1.5 is the mean
So C is the answer