Decay means the value gets smaller.
In order for a value to get smaller you need to multiply by a value less than 1
The first equation uses 0.5 which is less than 1
The second equation uses 0.4 which is less than 1
The third equation does not have a decimal and uses 4, which is greater than 1, so cannot get smaller.
The fourth equation uses 0.5 which is less than 1
The answer would be the 3rd one.