The correct answer is C) Altostratus. The altostratus cloud type is indicated by the number 4 in the illustration D039NG. Altostratus clouds are a mid-level cloud type that appear as a grayish, sheet-like layer covering the entire sky. The other answer choices are incorrect because: A) Altocumulus are mid-level, patchy clouds; B) Cumulus are low-level, puffy clouds; and D) Cirrostratus are high-level, sheet-like clouds. The illustration clearly depicts the altostratus cloud type with the number 4.