Which of following is true presentation of unit of silicate  $9/4$ oxygen shared per tetrahedra in double chain silicate?

Consider the give structure present in option 2 where the red dots shows the number of oxygen atoms shared by per tetrahedral unit of the given silicate.

In the above image, there are four tetrahedral units, out of which three tetrahedral units shared two O atoms and one tetrahedral unit shares 3 oxygen atoms. The average number of O atoms shared by each tetrahedral unit is calculated as:

$\begin{array}{ccc}\mathrm{Average} \mathrm{number} \mathrm{of} \mathrm{O} \mathrm{atoms}& =& \frac{2+2+2+3}{4}\\ & =& \frac{9}{4}\end{array}$