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**Hint:**

Electrons and protons consist of negative and positive charge respectively. Protons along with the neutrons form nuclei of an atom. James Chadwick discovered neutrons. The representation of any element is done by placing the mass number on the upper subscript and atomic number on the lower subscript.

**Complete step by step answer:**

-Isotones are those elements who have the same number of neutrons in the orbit.

-To calculate the number of neutrons we have to apply the formula that is:

Number of neutrons = Atomic mass - atomic number

-So, now we will apply this formula to all the element and we will get:

-In Argon, atomic mass is 40 and the atomic number is 18 so the number of neutrons will be

= 40 - 18 = 22

-In Calcium, atomic mass is 42 and the atomic number is 20 so the number of neutrons will be

= 42 - 20 = 22

-In Scandium, atomic mass is 43 and the atomic number is 21 so the number of neutrons will be

= 43 - 21 = 22

-In Scandium, atomic mass is 41 and the atomic number is 21 so the number of neutrons will be

= 41 - 21 = 20

-So, the pair of isotones are $\text{Ar}_{18}^{40}$, $\text{Ca}_{20}^{42}$and $\text{Sc}_{21}^{43}$ because they have an equal number of neutrons that is equal to 22.

**Therefore, option A, B and C are the correct options.**

**Note:**

Isotopes are those elements who have same no of electron pairs, for example, ${}_{6}^{12}\text{C, }{}_{6}^{13}\text{C and }{}_{6}^{14}\text{C}$ they have the same number of electrons but different mass i.e. 12, 13 and 14 respectively. Isobars are those elements which have the same mass number but a different atomic number. For example, $\text{C}{{\text{a}}^{20}}\text{ and }{{\text{K}}^{20}}$.

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