Explanation:

- The length of a cuboid is given as 20 meters.

- The breadth of the cuboid is 5 meters.

- The height of the cuboid is 4 meters.

- To find the diagonal of the cuboid, use the formula:

$$

\text{Diagonal} = \sqrt{l^2 + b^2 + h^2}

$$

where \( l \) is the length, \( b \) is the breadth, and \( h \) is the height.

- Substitute the given values:

$$

\text{Diagonal} = \sqrt{20^2 + 5^2 + 4^2} = \sqrt{400 + 25 + 16} = \sqrt{441} = 21 \, \text{m}

$$

- Evaluating the options:

- Option 1: 12 m

- Option 2: 21 m

- Option 3: 18 m

- Option 4: 24 m