To determine the maximum number of slabs sized \(20 \times 20 \times 5 \text{ cm}^3\) that can be cut from a cubic block of marble \(1 \times 1 \times 1 \text{ m}^3\) (which is \(100 \times 100 \times 100 \text{ cm}^3\)), while avoiding the planar fracture, follow the steps below:

1. First, calculate the volume of the cubic marble block: \(100 \times 100 \times 100 \text{ cm}^3 = 1,000,000 \text{ cm}^3\).
2. Next, calculate the volume of one slab: \(20 \times 20 \times 5 \text{ cm}^3 = 2,000 \text{ cm}^3\).
3. Calculate the theoretical maximum number of slabs that can be cut if there was no fracture: \(\frac{1,000,000}{2,000} = 500\) slabs.
4. Due to the planar fracture shown in the diagram, some of the slabs cannot be obtained without including fractured material. The arrangement in \(x, y,\) and \(z\) directions should be considered.
5. The optimal arrangement that avoids the fracture while allowing maximum slabs can involve slicing parallel to the fracture such that slabs are not wasted. In many configurations, experience shows us 100 slabs have to be discarded due to the fracture.
6. Thus, the maximum number of intact slabs that can be obtained is \(500 - 100 = 400\).

Therefore, the correct answer is 400.