Answer:
D. Al^3+
Explanation:
All the other elements given have the same number of elecetron
K has 19 electrons, since it is given as K+, it means that K has lost one electon, therefore K has 18 electrons now
Cl has 17 electrond, since it is given Cl^1- it means Cl has gained one electron , therefore now Cl has 18 electrons
Ar normally has 18 electrons
Al has 13 electrons, since it is given has Al^3+ it means Al has lost 3 electons, therefore now Al has only 10 electrons
In oder to answer this question easily it is must to know first 20 elements of the Modern Peroidic Table
