Which is correct to say, “the yolk of the egg are white” or “the yolk of the egg is white”?

Yolk OF the egg is singular, so its "is". 

 The yolk of the egg is white

Correct Order:

D. The lava and sand formation that resulted from the eruption are barren with few nutrients.

C. Vascular plants included sea lyme grass and oyster plants appear.

B. Mosses, lichens, and plants that are adapted to be dispersed by the wind or sea and able to grow in  the sand appear on the island.

A. Dwarf willow trees begin to grow on the island.

Explanation:

The very first vascular plant was observed on Surtsey's northern shore in 1965 during the spring while mosses were evident in 1967 and lichens were first identified in 1970 on Surtsey lava. Plant expansion on Surtsey was carefully examined, particularly as vascular plants had much greater value in vegetation growth than mosses, lichens and fungi.But now lichens and mosses are covered in most area of the islands.Twenty species of plants were recorded at some time or another during the very starting twenty years of the island but only ten were formed in the nutritional-poor sandy soil. As birds started breeding on the island, soil conditions were improving, and more varieties of vascular plants could survive.