(a) is the correct option as the incomplete sentence says that 'the Universe has not been eternal' or it will not been lasting or existing forever. So, (a) correctly mentions the usage of 'limited lifespan'. (b) & (c) is the repetition of the last part (the Universe has not been eternal) of the incomplete sentence. Our religious beliefs also says the same thing. So usage of 'however, on second glance' is absolutely wrong. (d) is absurd as the passage does not talk about energy at all. (e) is wrong as it negates the basic premise of the passage that the Universe has existed earlier as well, although destroyed and further created.