The conjunction not only/but also is used to connect balanced words or phrases in a sentence. One should note that the same parts of speech for each phrase must be used. This conjunction is used when the sentence provides two pieces of information. The first piece of information should come after 'not only,' and the second piece of information should come after 'but also’. Example : She not only likes jewelry, but she also likes dresses. In the given sentence, but should be used instead of and. Therefore, the correct option is 1. Correct sentence - Not only can coffee make you smarter in the short term, but it may also protect your brain in old age.