In petrology, rocks are classified based on their origin, texture, and mineral composition. Plutonic rocks, also known as intrusive igneous rocks, form underground from the cooling and solidification of magma. Volcanic rocks, or extrusive igneous rocks, form on the Earth's surface from the cooling and solidification of lava. Each type of plutonic rock has a volcanic equivalent based on their mineral composition.

Let's examine the given pairs:

1. Gabbro-Trachyte: Gabbro is a coarse-grained plutonic rock that is rich in iron and magnesium, typically forming from mafic magma. Its volcanic equivalent is Basalt, not Trachyte. Trachyte is a volcanic rock more similar in composition to Syenite.
2. Syenite-Andesite: Syenite is a plutonic rock containing a high percentage of alkaline feldspar and little or no quartz, similar to Trachyte. Andesite, the volcanic equivalent of Diorite, is not the correct match for Syenite.
3. Granite-Rhyolite: Granite is a coarse-grained plutonic rock composed mainly of quartz, feldspar, and mica. Its volcanic equivalent is Rhyolite, which has a similar mineral composition but with fine-grained texture due to rapid cooling on the Earth's surface. Therefore, this is the correct pair.
4. Granodiorite-Basalt: Granodiorite is similar to Granite but contains more plagioclase feldspar. Its volcanic equivalent would be Dacite, not Basalt, which is the volcanic equivalent of Gabbro.

Thus, the correct pair of plutonic rock and its volcanic equivalent is Granite-Rhyolite.