Main sentence- 'his discovering of methane gas' is the first red alert, 'what would become known as' is the second red alert.
eliminate A
Option B-his having discovered methane gas(what is this?), Alessandro Volta, the Italian physicist, developed the first electric battery, which will later become known as the voltaic pile(how are you predicting the future?)? Eliminate
Option C- his discovery of methane gas(seems right), Italian physicist Alessandro Volta developed the first electric battery, later known as the voltaic pile(this is good too)- Hold on to this
Option D- discovered methane gas(put it in the original sentence- nearly 25 years after discovered methane gas- this is wrong) , Italian physicist Alessandro Volta's discovery of the first electric battery, later known as the voltaic pile- eliminate
Option E- his discovery of methane gas, Alessandro Volta was an Italian physicist who developed the first electric battery, which would later be known as the voltaic pile- eliminating this because it's redundant and I've got a better option which is C