Indicate the ground on which a complaint under the Consumer Protection Act, 1986 shall be invalid?

The correct answer is Option B: A service provided free of cost.
Let's break down why:
The Consumer Protection Act is designed to protect consumers who purchase goods or services for a price.
Option A: A product having short weight is a valid ground for complaint because you paid for a certain amount and didn't receive it.
Option C: A misleading advertisement given in newspaper is also a valid ground because it influenced your decision to purchase, and the product didn't live up to the claims.
Option B: A service provided free of cost, however, is generally not covered. The Act usually applies when there's a transaction involving payment. If you didn't pay for the service, you typically can't file a complaint under this Act.
Option D: None of the above is incorrect because options A and C *are* valid grounds.