By what per cent should C increase its revenue in 2017 to make it 10% more than the average revenue of all the firms in 2016?

The average revenue of all three firms in 2016 is $$\dfrac{4230+4075+3965}{3}=4090$$

Firm C's revenue in 2016 is 3965 cr

As the firm has to make 10% more than the average of 2016, the revenue will be 

$$4090\times\left(1+\dfrac{10}{100}\right)=4499$$ crores

Assuming the percentage increase in C's revenue is $$x$$

So, $$3965\times\left(1+\dfrac{x}{100}\right)=4499$$

or, $$\left(1+\dfrac{x}{100}\right)=1.1347$$

$$x=13.47\%$$

Hence, the answer is 13.47%