Final answer:
Extrusive igneous rocks have the lowest melting temperatures among igneous rocks due to their rapid cooling and fine-grained structure, in contrast to plutonic rocks, which have higher melting temperatures and coarser crystals.
Step-by-step explanation:
Following Bowen's Reaction Series, the rock type that generally has the lowest melting temperatures among the igneous rocks would be the extrusive igneous rocks. This is due to the rapid cooling process these rocks undergo as they are formed from lava erupted onto the earth's surface. Characteristics of extrusive igneous rocks include rapid cooling, fine grain size, and often a lack of visible crystal growth. In contrast, plutonic igneous rocks, which cool slower deep below the earth's surface, have higher melting temperatures and are typically coarse-grained with well-formed crystals.