If the statement 'No fishes are mammals' is given as false, then which of the following proposition can be immediately inferred to be true?

The proposition immediately inferred true is "Some fishes are mammals".

"No fishes are mammals" is a universal negative (E), and here it is taken as false.

The contradictory of an E proposition is the particular affirmative (I).

When a proposition is false its contradictory must be true, so "Some fishes are mammals" is true.

This uses the contradictory relation of E and I on the square of opposition.