Final answer:
LIGO and Virgo have confirmed black hole mergers via gravitational waves, but aren't designed to detect potential companion black holes to Sag A*, the supermassive black hole at the Milky Way's center.
Step-by-step explanation:
The observatories that have confirmed the merging of black holes but are not sensitive enough to sense if our galaxy had a companion black hole to Sagittarius A* (Sag A*) are a) LIGO and b) Virgo. Both LIGO and Virgo are ground-based gravitational-wave detectors that have detected mergers of black holes with masses up to about 100 times the mass of the Sun. However, they are not designed to detect the presence of a companion black hole to the supermassive black hole at the center of our galaxy. Electromagnetic observations have not been detected from these mergers, underscoring their unique capability to observe gravitational waves under the framework of multi-messenger astronomy.
Observatories like c) IceCube and d) ALMA are not involved in the detection of gravitational waves. IceCube is a neutrino observatory, and ALMA (Atacama Large Millimeter/submillimeter Array) is an astronomical observatory that uses radio telescopes to observe the universe. Thus, options c) and d) are not correct in the context of this question.