To determine who is the shortest among Gianni, Giorgio, and Giacomo, let's analyze the given information step by step:
- Gianni is taller than Giacomo.
- Giorgio is taller than Gianni and shorter than Giorgio.
From the given information, we can conclude the following:
- Giorgio is taller than Gianni.
- Giacomo is shorter than Gianni.
Therefore, the shortest among Gianni, Giorgio, and Giacomo is Giacomo.
The correct answer is option D) Giacomo.