The valency of an element refers to its tendency to form chemical bonds with other atoms. It indicates the number of electrons an atom can gain, lose, or share to achieve a stable electron configuration.

Option 1: Oxygen has a valency of two. It tends to gain two electrons to achieve a stable configuration.

Option 2: Lithium has a valency of one. It tends to lose one electron to achieve a stable configuration.

Option 3: Carbon has a valency of four. It can either gain or lose four electrons or form four covalent bonds to achieve stability.

Option 4: Boron has a valency of three. It tends to share or gain three electrons to reach a stable electron configuration.

Based on this information, the correct answer is option 4, Boron. It has a valency of three, meaning it tends to share or gain three electrons to achieve stability.