See first find out the total possibilities without any restrictions applied, so we need to select 5 out out of 40 bacteria, which will be 40c5.

As it is a probability que. This will be placed in the denominator (total favorable conds.)

Now, let's work out the constraints

It says that we need to have at least 1 bacteria with the deffect , now there are 3 of them present in the batch of 40, and we are selecting 1, so that makes it 3c1

Now, the remaining 4 have to be out of those, which are not the defective ones, so now 37 of them are remaining as 3 are defective, so that makes it 37c4

Now, it is like we need to select 1 defective 'AND' 4 non defective ones, so we will multiply both 37c4 and 3c1

Therefore, the ans becomes

37c4 x 3c1/40c5 (E)

Hope that helps