Rigorous study habits are important for developing intelligence, but it is a healthy and active physical regime also stimulates the brain.
a) intelligence, but a healthy and active physical regime it is that
b) intelligence, but then also a healthy and active physical regime
c) intelligence but it was a healthy and active physical regime
d) intelligence, but a healthy and active regime
e) no improvement or correction is required

D) intelligence, but a healthy and active regime.

Explanation: a coordinating conjunction is a conjunction placed between words, phrases, clauses, or sentences of equal rank (but, and, or). When we use a coordinating conjunction, we should put a comma before it. From the given options, the one that corrects the given sentence is the corresponding to option D, because it mantains the comma before "but" and it clarifies the sentence: "Rigorous study habits are important for developing intelligence, but a healthy and active regime also stimulates the brain."

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d

explanation: