No entiendo porque se dice "al que" y no "que".
Freeform Writing Exercise B2
You can also say "El cerebro es un órgano poderoso que podemos transformar...". It will be added as another possible correct answer.
For your information, the use of "a" here [al que...] is difficult to justify if we think about the general rule about using "a" with transitive verbs and direct objects. We normally use the preposition "a" with direct objects that are "people". The R.A.E (Spanish Royal Academy) says that sometimes we "personify" the objects by using the preposition "a" when they are direct objects. This would justify the use the preposition here, in my opinion.
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