For the first time, research on nerve cells was carried out on this organism

The correct option is Giant squid.

In the 1930s and 1940s, pioneering neuroscience research on the mechanism of nerve impulses was carried out using the giant axon of the giant squid (Loligo pealei).
Due to the exceptionally large diameter of this axon (typically around 0.5 to 1 mm, which is about 100 to 1000 times larger than human axons), scientists Alan Hodgkin and Andrew Huxley were able to insert microelectrodes inside the nerve fiber to measure its electrical properties.
This research led to the discovery of the ionic basis of action potentials, for which they were awarded the Nobel Prize in Physiology or Medicine in 1963.