H20
carbon
Carbon.
Water would be the most abundant substance but if you're asking about elements then oxygen would make up the greatest percentage of body mass.
Water is the single most abundant compound in living things.
Water is the most abundant compound in living organisms.
Water is the single most abundant chemical substance in the body, making up about 60% of an adult's total body weight.
Water
Single most (what?) substance ---common? ---important? ---fragile? This question is not formed properly and thus can not be answered
Copper wire is a substance because it is made up of a single type of material, which is copper. It is not a mixture of different substances.
Living in the Material World
Yes, the genetic material in living organisms can be double stranded, as seen in DNA, or single stranded, as seen in RNA.
A single substance of matter refers to a material that has a uniform and definite composition, characterized by distinct physical and chemical properties. This can include elements, like gold or oxygen, which consist of only one type of atom, or compounds, like water or carbon dioxide, which are made up of two or more types of atoms chemically bonded together. In essence, a single substance is a homogeneous material that cannot be separated into different components by physical means.