This igneous rock contains flattened pieces of pumice and small crystals of quartz and feldspar. what type of eruption probably formed this rock?

There are various types of igneous rocks from phaneritic and pegmatite to coarsely crystalline. The welded texture is best described as volcanic ash and pumice, when still hot it can be compacted by overlying materials, becoming a hard rock. This would mean that volcanic eruptions would be most likely to form this rock.