Thiobacillus is a group of bacteria helpful in carrying out nitrification, denitrification, nitrogen fixation, or chemoautotrophic fixation?

Correct option is D. Denitrification
Thiobacillus denitrificans is chemolithoautotrophic bacteria that denitrifies nitrate during oxidation of FeSx compounds. So, the correct answer is 'Denitrification'