Explanation:
Here is a tree diagram showing all possible shirt, pant, and shoe combinations for John:
```
+-------------+
| Shirts |
| (3 options) |
+-------------+
|
|
v
+-------------+
| Pants |
| (4 options) |
+-------------+
|
|
v
+---------------------------+
| Shoes |
| (2 options: boots or dress)|
+---------------------------+
|
|
v
+-----------------------------+
| All Possible Combinations |
| (3 x 4 x 2 = 24 options) |
+-----------------------------+
```
The tree diagram starts with the three options for shirts, then branches out to the four options for pants, and finally to the two options for shoes (boots or dress shoes). Multiplying the number of options at each stage gives us the total number of possible combinations, which is 3 x 4 x 2 = 24.