If a number is divisible by 8,then the number obtained by the last three digits of the number should be divisible by 8.

So, 6a2 should be divisible by 8.

Here,a can be any value from 0,1,…9.

When a = 0,6a2 = 602,which is not divisible by 8.

When a = 1,6a2 = 612,which is not divisible by 8.

When a = 2,6a2 = 622, which is not divisible by 8.

When a = 3,6a2 = 632,which is divisible by 8.

Hence, the least value of a is 3,which makes the number 91876a2 divisible by 8.