In a row of girls, Rimmi and Mimmi occupy ninth place from right end and tenth place from left end respectively. If they interchange their positions then Rimmi and Mimmi will occupy seventeenth place from right end and eighteenth place from left end respectively.

How many girls are there over all in all the given row :

Let the number of girls in between Rimmi and Mimmi be $$x$$. 

It is given that Rimmi and Mimmi occupies ninth and tenth place from the right end and left end respectively. 

Number of girls to the right of Rimmi = 8 (as she occupies 9th place from the right end)

Number of girls to the left of Mimmi = 9 (as she occupies 10th place from the left end)

The initial arrangement : $$9,\ M,\ x,\ R,\ 8$$ 

Now, both Rimmi and Mimmi interchanged their positions. 

Now, the new arrangement: $$9,\ R,\ x,\ M,\ 8$$

Now, Rimmi is 17th place from the right. It means that Rimmi has 16 girls to her right. 

$$8+1+x=16$$

$$x=16-9=7$$

We have 7 girls in between Rimmi and Mimmi. 

Total number of girls in the arrangement = $$8+1+7+1+9=26$$

Hence, we have a total of 26 girls in the given row. 

Option C.