Final answer:
A black hole would need to be enormously massive, exceeding galactic mass, to have the density of water, due to the inverse relationship between mass and density for black holes. Hence, the answer is (c) Greater than a galactic mass.
Step-by-step explanation:
The question asks how massive a black hole would need to be to have the density of water. To find a black hole with the density of water, we need to consider the characteristics and scales of black holes. The density of water is approximately 1000 kg/m³. A black hole's density decreases as its mass increases, due to the radius of the event horizon expanding at a greater rate than mass is added.
A supermassive black hole, such as the one at the center of our galaxy, Sagittarius A*, has a mass of around 4.6 million solar masses and a radius much larger than what would be needed to have the density of water.
Therefore, we can conclude that to answer the question, the black hole would need to be enormously massive, way beyond the mass of smaller astronomical objects like Earth or neutron stars, and even greater than the mass of many stars combined, possibly even exceeding galactic mass. Hence, the answer is (c) Greater than a galactic mass.