Glassy is a term that would suggest that the rock is igneous.
Explanation:
Out of the three terms, ''glassy'' is the one that for sure suggests that the rock would be of igneous type. We can be so sure because glassy rocks occur only among igneous rocks, while they are not present among metamorphic and sedimentary rocks.
The glassy igneous rocks are formed by lava that has cooled off extremely quickly. The cooling has been so rapid that there hasn't been any time for crystals to form. On top of it, the silica content is very high, so the glass-type rocks form, being very smooth and curved, as well as shiny. The most recognizable of the glassy igneous rocks is the obsidian.
