Which algae are responsible for causing red tides that can kill thousands of fish?
chlorophyta
rhodophyta
phaeophyta
pyrrophyta

Fourth option is correct.

Step-by-step explanation:

Pyrrophyta is the algae which is responsible for causing red tides that can kill thousands of fish.

As we know that Pyrrophyta is a division category of algae consisting yellowish green to golden brown algae.

They are mostly unicellular which form starch and its compounds, and oil as food reserves.

And it includes dinoflagellates which actually cause red tides that kill so many fish.

Hence, fourth option is correct.