Identify the pair of atoms of elements as isotones from the...

Isotones are atoms of different elements having the same number of neutrons.

The number of neutrons is given by $A - Z$, where $A$ is the mass number and $Z$ is the atomic number.

For ${}_{5}\textB^{11}$, number of neutrons $= 11 - 5 = 6$

For ${}_{6}\textC^{12}$, number of neutrons $= 12 - 6 = 6$

Since both have the same number of neutrons, they are isotones.

Answer: ${}_{5}\textB^{11}$, ${}_{6}\textC^{12}$