Which of the following bird species is the smallest?

The correct answer is Option (4) - Hummingbird

Hummingbirds are members of the Trochilidae family of birds. They are among the smallest bird species, with the majority of species measuring between 7.5 and 13 cm (3-5 in).

The 5-cm Bee Humming bird, the tiniest extinct bird species, is a hummingbird.

Since they are the smallest birds, they can fly both forward and backward.