Final answer:
A sample of water at 20º C contains both covalent and hydrogen bonds, due to the polar nature of the covalent bonds in water and the attraction between the partial charges of different water molecules.
Step-by-step explanation:
A sample of water at 20º C contains C) both covalent and hydrogen bonds. Water molecules are made up of hydrogen and oxygen atoms connected by polar covalent bonds. The shared electrons comprising an O-H bond are attracted to the more electronegative O atom, giving it a partial negative charge. The H atoms exhibit partial positive charges. These water molecules can also form hydrogen bonds with each other, where the positive part of one water molecule is attracted to the negative part of another due to the polar nature of the covalent bonds within them.