C is not the answer because we do not care if they are replacing a competitors computer or a PXC model.

We only care that more profits are made.

That means you need to sell more computers of the higher profit - since the same number of computers overall were sold so no way to get more money with greater volume, you need to get more profit per computer.

Well if first-time buyers yield a smaller profit we need fewer of those and more of the upgrade type buyers. That is what choice D gives you.

Total PXC computer sold=[New PXC models (has low profit margin)] +[Computers purchased to replace with Powerful PXC computer(has higher profit margin)]

T= A+B

As T is fixed for last and previous years. In D, it is told that A is smaller in last year. As A+B is constant, does it shows B i.e. number replaced computers(with high profit) margin has incresed?

hence D is correct choice.