The number of different kinds of mammals doesn’t impact the significance of the evidence. And even if it did, (A) might strengthen by providing additional reason to think birds faced competition from mammals.

This undermines the assumption that the indigenous land mammal recently discovered actually competed with New Zealand’s birds. If it was extinct before the birds came around, it’s not evidence that birds faced competition from land mammals.

The argument concerns competition between birds and land mammals. Reptiles and insects are not mammals and are therefore irrelevant.

We know NZ has a rich and varied bird population, which, in connection with (D), would imply that it doesn’t have a rich and varied population of land mammals. But this doesn’t affect anything concerning the recently discovered mammal and how it might have affected birds.

Whether other island countries also had indigenous land mammals doesn’t affect whether the recently discovered land mammal on New Zealand implies that birds had competition from land mammals.