The ratio of boys and girls passed from school Y in 2017 is 7 : 8 and that of boys and
girls passed from school X in 2019 is 12 : 13. The percentage of girls appeared from
school Z in 2020 is 80%. The total number of girls passed from schools X in 2019 and
from Y in 2017, is what per cent less than the number of girls appeared from school Z
in 2020 (correct to one decimal place)?

A total of 225 students passed from School Y in the year 2017. 
We are given that among these students, the boys and girls are in the ratio of 7:8
This would give us the girls passing from school Y in 2017 to be $$\frac{225}{15}\times\ 8=120$$, and the boys passing from school Y in 2017 to be $$\frac{225}{15}\times\ 7=105$$

Next, we are given the ratio of boys and girls that passed from school X in the year 2019 as 12:13
The total number of students that passed from X in 2019 is 280; this would give us the number of boys and girls passing in that year to be $$\frac{275}{25}\times\ 12=132$$ and $$\frac{275}{25}\times\ 13=143$$ respectively.

The total number of girls passing from X and Y would be: 120+143 = 263

We are given that in 2020, 80% of the students who appeared were girls, giving us this value to be $$\frac{80}{100}\times\ 400=320$$

We are asked how many percentages is 263 less than 320; this value would be $$\frac{320-263}{320}\times\ 100=17.81\%$$

Therefore, Option C is the correct answer.