The presence of wings in flightless birds, such as the ostrich, can be best interpreted as:

This shows that they share their ancestry with a flying bird.

Explanation:

The mass extinction of the population of dinosaurs heavily contributes to the fact that why the birds like ostrich remained flightless. With the extinction of dinosaurs, a lot of Freeland was there to be accounted for and because of this various aerial animals like birds started to amend with the land life.  

Thus, the birds like ostrich started to gain in size and got amended with their new life, and these birds got bigger in size, they lost the tendency to fly. It is a general misconception that the ancestor of the ostrich was a flightless bird, however, the ancestor of ostrich, in fact, was a flying bird. The evolution of ostrich made it lose the tendency to fly, in fact, it forgot how to fly.  

the answer is d. chordata

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