what missing code should be added to this class to ensure that calling factorial$(5)$ returns $120?$ class TEST feature factorial$($n: INTEGER$)$: INTEGER do if n $<=1$ then Result :$=1$ else Result :$=$ n $*\_\_\_$ end end end

a$.$

$1$

b$.$

factorial$($n $-2)$

c$.$

n $*$ factorial$($n $-1)$

d$.$

factorial$($n $-1)$