Solution:

The answer is (D). By symmetry, each of Marco, Fabrizio and Giovanni must spend the same amount of time on defense for the mathematicians' team, which therefore turns out to be one third of the 24 hours of the tournament, that is, 8 hours. Alternatively, we can observe that there will be $3 \cdot 2 \cdot 4 \cdot 3$ matches in total (3 ways to choose the mathematicians' forward, 2 for the defender, 4 for the physicists' forward and 3 for the defender); the matches in which Marco is defender are $2 \cdot 4 \cdot 3$, and thus the hours he spends on defense are $\frac{2 \cdot 4 \cdot 3}{3 \cdot 2 \cdot 4 \cdot 3} 24=\frac{24}{3}=8$.