If the statements "All whales are mammals" is given as false which of the following statement can be immediately inferred to be true?

The statement immediately inferred true is "Some whales are not mammals".

"All whales are mammals" is a universal affirmative (A), taken here as false.

The contradictory of an A proposition is the particular negative (O).

When a proposition is false its contradictory must be true, so "Some whales are not mammals" is true.

This uses the contradictory relation of A and O on the square of opposition.