I would choose option A for this very subjective question, as "handiness" would capture the suitability of form and ease of access. Which are features present in the construction of the birds nest, that represent its perfect convenience, along with beauty and contexture. And not only the form.
But taking the answer given, and assuming that the whole Montaigne piece focuses in the material aspects of nature´s creations, not the functional aspects, "Suitability of form" is the right answer, as it already relates to the next example of nature´s perfection: the spider web.
