The following statements are correct: (1) Nostoc flagelliforme belongs to eukaryote; 2) There are both autotrophic and heterotrophic viruses in prokaryotes; 3) The virus is the simplest.

Nostoc flagelliforme is a prokaryote, which is wrong;

② The cyanobacteria and nitrifying bacteria in prokaryotes belong to autotrophs, and most of them belong to heterotrophs, which is correct;

③ Cell is the most basic structural level of life system, ③ Wrong;

④ A single-celled organism is an individual, which can complete all kinds of life activities. 4 correct;

⑤HIV is a virus without cell structure and cell membrane, ⑤ wrong;

⑥ Ecosystem is composed of biological community and inorganic environment, ⑥ Correct;

⑦ Paramecium is a unicellular organism, so it belongs to both cell level and individual level, ⑦ Correct;

It is wrong that there is no "system" at the structural level of plant life system.

So choose: a.