A book costs Rs.35 and a pen costs Rs.20. A person has purchased less number of books than the number of pens by spending maximum amount out of Rs. 350, that he has. The amount left unspent (in rupees) is

Let say,he bought x number of pens and y number of books.

each pen cost 20 rs.and each book cost 35 rs.

So,it will cost a total of $$(20x+35y)$$rs.

So, According to the question,

$$350\ge(20x+35y). $$

by using trial and error method,

if we put x=10 and y=4 then,

$$20x+35y=200+140=340$$

which is less than 350.

But if we put x=14 and y=2 then,

$$20x+35y=280+70=350.$$

So, we can utilize the 350 Rs. full only when he purchase 14 number of pen and 2 number of books.

So, no rupees will left with him.

A is correct choice.