Discussion
Assertion (A) : Granite has larger crystals than basalt.
Reason (R) : Basalt has larger content of metallic minerals.
Which one of the following is correct ?
Answer: B
Both statements are true but the Reason does not explain the Assertion, so the answer is that (R) is not the correct explanation of (A).
Granite is an intrusive (plutonic) rock: its magma cooled slowly deep in the crust, giving the minerals time to grow large crystals. Basalt is an extrusive rock: lava cooled quickly at the surface, so its crystals are fine. Crystal size is therefore explained by the rate of cooling.
The Reason is also true, since basalt is a mafic rock rich in iron and magnesium minerals while granite is felsic and rich in quartz and feldspar. But mineral composition does not decide crystal size, so the Reason is a true statement that does not explain the Assertion. That rules out the first option, and both one-false options.
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