P is twice as efficient as Q. Q takes 12 days to complete a job. If both of them work together, how much time will they take to complete the job?

P is twice efficient than Q.

It means that Q take 2 times of P's time to complete the job.

So, P will take $$\left(\frac{12}{2}\right)=6$$ days to complete the job.

So, P can do 1/6 part of the job in 1 day and Q can do 1/12 part of the job in 1 day.

So,they together can do $$\left(\frac{1}{6}+\frac{1}{12}\right)=\frac{3}{12}=\frac{1}{4}$$ part of job in 1 day.

So, they together will take 4 days to complete the job.

C is correct choice.