Correct answer is C.
Q can finish the task in $25$ days, working alone for $12$ hours a day.
What can we learn from this line??
That Q, in $25\times 12$ hours can complete the work alone.
That is, his rate of doing work per hour is $\frac{1}{25\times 12}=\frac{1}{300}.$
R can finish the task in $50$ days, working alone for $12$ hours per day.
Now, similarly, R's per hour work is $\frac{1}{600}.$
Now, Q has worked for $5$ days of $12$ hours ($60$ hours) and R for $7$ days of $18$ hours ($126$ hours).
We finally know, how many hours both worked, and their capacity for an hour.
Ratio of work done by Q and R after $7$ days from the start of the project
$\qquad\qquad=\frac{\text{Work done by Q}}{\text{Work done by R}} = \frac{60\times \frac{1}{300}}{126\times \frac{1}{600}}=\frac{20}{21}.$
