Biologists working in Spain say that their discovery of teeming life in a highly acidic river may not only broaden the search for life, or for evidence of past life, no other planets but also show that a number of forms of microscopic life can adapt to conditions that scientists have long thought hostile to all but the hardiest bacteria.

A. show that a number of forms of microscopic life can adapt to conditions that scientists have long thought hostile to all but the hardiest bacteria
Broaden is plural and hence the show is plural which is correct.
B. may show that a number of forms of microscopic life is capable of adapting to conditions that scientists have long thought hostile to all bacteria but the hardiest ones
may broaden and may show is not correct especially if may comes after but also.
C. shows a number of forms of microscopic life to be capable to adapt to conditions that scientists have long thought had been hostile to all but the hardiest bacteria
may not only broaden (plural) should be accompanied with show and not shows (singular)
D. showing that a number of forms of microscopic life is capable of adapting to conditions that scientists have long thought had been hostile to all but the hardiest bacteria
may not only broaden and showing are not parallel
E. showing that a number of forms of microscopic life can adapt to conditions that scientists have long thought hostile to all bacteria but the hardiest