**Explanation:**

To understand why the correct answer is option 'A' (s-orbital), let's first define what a directional orbital is.

**Directional Orbitals:**
Directional orbitals are those orbitals that have a preferred direction in space along which the probability of finding an electron is higher. In other words, the electron density of a directional orbital is not symmetrically distributed around the nucleus, but rather concentrated in specific regions.

Now, let's consider the different types of orbitals mentioned in the options and determine which one fits the definition of a non-directional orbital.

**a) s-orbital:**
An s-orbital is a spherical-shaped orbital that is symmetrical in all directions. The electron density of an s-orbital is evenly distributed around the nucleus, without any preferred direction. Therefore, it is considered a non-directional orbital.

**b) f-orbital:**
F-orbitals are complex-shaped orbitals that have multiple lobes and nodes. Unlike s-orbitals, f-orbitals do have preferred directions in space. They have more complex shapes and orientations, making them directional orbitals.

**c) d-orbital:**
D-orbitals, similar to f-orbitals, have complex shapes and orientations. They also have preferred directions in space and are therefore considered directional orbitals.

**d) p-orbital:**
P-orbitals are dumbbell-shaped orbitals that have two lobes separated by a nodal plane. The electron density is concentrated in these lobes and is distributed along an axis. P-orbitals have a preferred direction and are therefore considered directional orbitals.

Based on the above explanations, it is clear that the s-orbital (option 'A') is the only non-directional orbital among the options given.

The s orbital is nondirectional because it has spherical symmetry . Spherical symmetry means that the probabilityof finding an electron at a particular distance from the nucleus is same in all directions while in p orbital which has dumble symmetry is not possible hence it is directional.