An ovation is prolonged, enthusiastic applause, so applause is necessary to an ovation. An outburst (choice a) may take place during an ovation; "bravo (choice b) may or may not be uttered; and an encore (choice d) would take place after an ovation.

Sustenance is something, especially food, that sustains life or health, so nourishment is the essential element.Water and grains (choices b and c) are components of nourishment, but other things can be taken in as well. A menu (choice d) may present a list of foods, but it is not essential to sustenance.

A purchase is an acquisition of something. A
purchase may be made by trade (choice a) or
with money (choice b), so those are not essential elements. A bank (choice c) may or may not be involved in a purchase.

All vertebrates have a backbone. Reptiles
(choice b) are vertebrates, but so are many
other animals. Mammals (choice c) are vertebrates, but so are birds and reptiles. All vertebrates (choice d) are animals, but not all animals are vertebrates.

Provisions imply the general supplies needed,
so choice b is the essential element. The other choices are byproducts, but they are not
essential.