Concept:A helioscope is the specialized instrument for safely viewing the sun.Explanation:A telescope is for observing distant objects, not safe for direct solar viewing.A helioscope is designed to observe the sun and its sunspots without damaging the eye.It was first used by Benedetto Castelli and later refined by Galileo Galilei.This makes it the most suitable instrument for the purpose.Answer:A. helioscope