The number of electrons and the group number of the given species are given below:
Ions
At. No
Group
Period
No. of e−s
S2−
16
16
3
18
K+
13
1
4
18
Ca2+
20
2
4
18
There ions are isoelectronic, therefore S2− have greater size due to higher effective nuclear charge (Zeff) followed by K+ then Ca2+ Hence, the correct order of decreasing ionic radii is Ca2+<K+<S2−