The atomic number of the element which does not usually form compounds because its outermost shell is full is a.8 b.16 c.18 d.19 e.20

Correct 18
The outermost shell is full when that shell reaches 2(n²) electrons distributed by holding-capacity of each layer, where n is the outermost shell number. The element with an atomic number of 18 has 18 electrons around it. For the first shell, it is full when it has two electrons [2(1²)= 2 on the first shell]. The second shell is full when it has six electrons [2(2²)= 2 on the first shell + 6 on the second]. The third shell is full when it has 8 electrons [2(3²)= 2 on the first shell + 6 on the second + 8 on the third shell]. 18 electrons fill up the outermost shell of the element.

Yes i think so not 100 percent sure though