Which bond is longest?

Hint: A bond formed by sharing of electrons is known as a covalent bond, while a bond formed by transfer of electrons is an ionic bond. Each molecule has different lengths of bonds. Generally, the bond between polar elements is shorter than that of nonpolar.

Complete answer:
A length of a bond is defined as the distance between the nuclei of two atoms bonded together. There are various factors that affect bond length of an atom. Some of them are:
-Size of the atom: A greater size atom has more length of bond.
-Multiplicity of a bond: Greater the multiplicity of the bonds less is the bond length. Like a compound with triple bond has less bond length, then the compound with double bonds, while a compound with single bonds has greatest bond length.
-Types of hybridization: S orbital has a small size, so, more is the s character, and less is the bond length.
The length of any bond is determined by the bond order. Bond order is the number of bonds between the atoms. If the bond order is high, then the bond energy is also high, which means that the bond length is short.
The longest bond is considered to be a carbon-carbon bond, present in diamond. Its length is 154 pm. It is the longest due to the three-dimensional structure of diamond, and the carbon atoms bonded through covalent bonds.
Hence, C-C bond in diamond is the longest bond, which is a covalent bond.

Note:
The length of a bond is measured in picometer (pm), and also in angstrom which is ${{A}^{0}}$, and $1\,{{A}^{0}}$= 100 picometers. Bond order is the number of bonds, like bond order of ${{H}_{2}}$ is 1 due to one covalent bond, while that of ${{O}_{2}}$ is 2 due to one double bond, and that of ${{N}_{2}}$is 3 due to a triple bond.