[tex]\green{ \underline { \boxed{ \sf{Average = \frac{Sum \:of \: observation}{Number \:of \: observation}}}}}[/tex]
[tex]\longrightarrow[/tex]Here, no of observation are 5
Now,
[tex]\begin{gathered}\\\implies\quad \sf Average = \frac{1+9+11+13+26}{5} \\\end{gathered} [/tex]
[tex]\begin{gathered}\\\implies\quad \sf Average = \frac{60}{5} \\\end{gathered} [/tex]
[tex]\begin{gathered}\\\implies\quad \sf Average = 12 \\\end{gathered} [/tex]
[tex]\longrightarrow[/tex]The Average of given set of numbers is 12 .
[tex]\therefore\: [/tex] Option A) is correct ✔️