Which of the substances in the chart has the least density?

            Option-D (Piece of Wood) is the correct answer.

Explanation:
                   Density is related to mass and volume as,

                             Density  =  Mass / Volume
For Wood,
                             Density  =  42.5 / 50

                             Density  =  0.85

You can calculate remaining densities by yourself. Among all the density of Piece of Wood is the least.

Spontaneity tell us the direction of the reaction, but not how fast it goes. gibbs free energy brings together enthalpy, entropy and temperature in one easy formula. g = h - ts. and, finally the sign of delta g is always negative for a spontaneous reaction.