Answer: The correct answer is -
Archaebacteria.
Archaebacteria is one of the three domains of life forms (Eubacteria and Eukarya) according to the three domain classification system of Carl Woese. It includes the primitive bacteria, which survive under extremely harsh environmental conditions (therefore also called extremophiles) such as hotsprings, hydrothermal vents, highly acidic and saline conditions.
As per the given information in the question, scientist were able to collect the organisms by dipping their microscope slides into the boiling hot springs.
Thus, they belong to Archaebacteria.