Answer: last choice
In general if we have first term a[1] and common difference d, such that
a[n] = a[n-1] + d
then the explicit formula is
a[n] = a[1] + d(n-1)
First choice has d=-4 so needs a minus sign in the explicit formula.
Second choice has a=-100 so needs a negative sign for the 100.
Third choice swaps a[1] and d, oops.
