How is hardness defined when identifying minerals?
ability to resist scratching
ability to scratch
density of a mineral
organization of crystals
size of crystals

Option (A)

Explanation:

The hardness of a mineral is defined as the ability to resist scratching. Weaker minerals easily get break down when scratching with a less hard mineral. Hard minerals are not easily scratchable.

In order to determine hardness, a scale was developed by Mohr, which is known as the Mohr's hardness scale. According to this scale, the hardness is measured on a scale from 1 to 10. They are given below in the increasing order-

Talc < Gypsum < Calcite < Fluorite < Apatite < Feldspar < Quartz < Topaz < Corundum < Diamond.

The least hard mineral is Talc and the strongest mineral is Diamond.

Thus, the correct answer is option (A).

organisms that lack symmetry are called asymmetrical and belong to phylum porifera/sponges.

organisms with specialized stinging cells belong to phylum cnidaria. stinging cells are called cnidoblast cells.

cnidarians show two morphological forms : polyp (sessile) and medusa (mobile)

filter feeding is observed in porifera members.  

alternation of generation is observed in coelenterata or cnidaria. there is alternation of asexual polyp form and sexual medusoid form. it is also referred as metagenesis.