**(C) 3 : 80**

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

Total number of pages in the Mathematics textbook for Class VI = 320 pages

The chapter ‘symmetry’ runs from page 261 to page 272

Number of pages contains symmetry chapter = 12

So, the ratio of the number of pages of this chapter to the total number of pages of the book is,

= 12/320

Divide both numerator and denominator by 12.

= 6/160

Again, divide both numerator and denominator by 2.

= 3/80

**Therefore, the ratio of the number of pages of this chapter to the total number of pages of the book is 3: 80.**