**(C) 27**

**From the question it is given that,**

On a shelf, books with green cover and that with brown cover are in the ratio 2:3

There are 18 books with green cover

**So, let us assume the common factor of 2 and 3 be y.**

Then, 2x = 18

X = 18/2

**Divide both numerator and denominator by 2.**

X = 9

**Therefore, the number of books with brown cover is = 3x = 3 × 9**

**= 27**