A can complete 50% of a job in 9 days and B can complete 25% of the job in 9 days if they worked alone. If they worked together how much of the job (in %) can they complete in 9 days?

A can do :

in 1 day = $$\left(\frac{1}{2}\times\frac{1}{9}\right)=\frac{1}{18}\ \ part\ of\ job\ .$$

B can do :

in 1 day = $$\left(\frac{1}{4}\times\frac{1}{9}\right)=\frac{1}{36}\ \ part\ of\ job\ .$$

So, in 1 day they together do = $$\left(\frac{1}{36}+\frac{1}{18}\right)=\frac{3}{36}=\frac{1}{12}\ \ part\ of\ job\ .$$

So, in 9 days they will do = $$\left(\frac{9\times100}{12}\right)=75\%\ \ \ of\ job\ .$$

C is correct choice.