Unsaturated carbon compounds burn in oxygen with excessive black smoke providing a flame with . . . . . . . . A. yellow B. red C. brown D. blue

The correct answer is C. brown.

Unsaturated carbon compounds burn in oxygen with excessive black smoke providing a flame with a brown color. This is because the incomplete combustion of these compounds produces carbon monoxide and soot, which are both black in color. The soot particles are suspended in the flame and give it a brown color.

Option A is incorrect because yellow flames are typically associated with the combustion of sodium-containing compounds. Option B is incorrect because red flames are typically associated with the combustion of carbon-containing compounds. Option D is incorrect because blue flames are typically associated with the combustion of hydrogen-containing compounds.