The argument is that “the habitats of wading birds will therefore inevitable decline if gardening continues to be so popular”.
A – Wrong
This does not necessarily need to be assumed as the text says that gardening has become the most popular. We need to find an assumption about the present not the future.
B – Wrong
The text assumes that since gardening has become the most popular hobby, gardeners will ignore the encouragement to use alternatives which will inevitable cause wading birds to die.
C – Wrong
Nothing is mentioned about environmentalists ignoring the declining habitats of wading birds.
D – Wrong
Again, we cannot infer that garden enters cannot sell cheap alternatives to peat just from the information provided to us.
E – Correct
nothing mentioned about the habitats of other animals.