Concept:Igneous rocks form when molten magma or lava cools and solidifies, so they are recognised by textures and features related to cooling conditions.
Explanation:Igneous rocks do not form in layers, so stratified layers are not an identifying feature.
They generally do not contain fossils because fossils are mainly found in sedimentary rocks.
A coarse texture appears in some intrusive igneous rocks, but it is not common to every igneous rock.
When lava cools very quickly, it can produce a smooth, glassy surface, as seen in rocks like obsidian.
This glassy appearance is a distinctive and easily observable feature of igneous rocks.
Hence, among the given options, glassy appearance is the best identifying feature.
Answer:A. glassy appearance