To determine the conditions under which groundwater occurs, we need to understand the characteristics of the aquifer systems:

1. Unconfined Aquifers: These are aquifers where the groundwater is directly open to the atmosphere through the spaces in the soil or rock. Groundwater can be easily recharged by rainwater. Hence, groundwater occurs here in unconfined conditions.
2. Semi-Confined Aquifers: These aquifers are partially covered by a semi-permeable layer, allowing some water to move in and out of the aquifer. This implies that groundwater can be stored under semi-confined conditions within these aquifers.
3. Confined Aquifers: These aquifers are sandwiched between two impermeable layers. The water within is under pressure, leading to occurrences of artesian wells. Therefore, groundwater is present under confined conditions in these types.

Given the explanation of each type, groundwater occurs under all the stated conditions:

• (a) Unconfined condition in shallow aquifers
• (b) Semi Confined aquifers
• (c) Confined aquifers

Thus, the correct answer is: (a), (b) and (c).

This confirms that all types of aquifers mentioned have the possibility of containing groundwater. Each type provides a different level of protection and recharge mechanism, which helps in understanding groundwater occurrence and management.