Basalt forms when magma cools very quickly. basalt
a. has small crystals
b. is an intrusive igneous rock
c. forms from heat and pressure
d. forms from contact metamorphism

A. has small crystals

Explanation:

Basalt is an example of extrusive igneous rocks. Igneous rocks can be extrusive or intrusive base on the formative environment.

Generally, igneous rocks are formed from molten magma. when  this magma solidified in the interior part of the earth it forms intrusive igneous rocks. Intrusive igneous rocks are usually coarse grained due to the slow process of solidification.

Extrusive igneous rocks are formed when this molten magma migrate to the earth surface and solidified. Due to the cooler nature of the earth surface the lava(molten magma on earth surface) rapidly solidified .This rapid solidification denies the crystals more time to form thereby making the crystals of extrusive igneous rocks small.

Basalt rocks are fine grain with small crystals . Notice the fined grain texture of basalt in the picture above .  

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