Q: Hexagonal close packed arrangement of ions is described as 

Explanation:
Hexagonal close packed (HCP) arrangement is one of the most efficient ways to pack spheres (such as ions) in a crystal structure. In HCP, each layer of ions is arranged in a hexagonal pattern, and each subsequent layer fits into the depressions of the layer below it to maximize packing efficiency.

The stacking sequence describes how these layers are placed on top of each other. In HCP, the stacking follows a repeating pattern of two distinct layers. This pattern can be represented as ABABA. Here’s what this means:

– A Layer: This is the first layer where ions are placed in a hexagonal arrangement.
– B Layer: The second layer is placed so that the ions fit into the pockets of the A layer.
– A Layer: The third layer repeats the first layer’s arrangement, aligning directly above the first A layer.

This ABABA sequence continues, maintaining the hexagonal close packing structure.

Looking at the options provided:

– Option A: ABC ABA suggests a two-part repeating sequence, which doesn’t correctly represent the HCP structure.
– Option B: ABC ABC represents the face-centered cubic (FCC) arrangement, not HCP.
– Option C: ABABA correctly represents the HCP stacking sequence.
– Option D: ABBAB does not follow a consistent pattern suitable for HCP.

Therefore, the correct answer is ABABA.