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

The question requires sentence correction "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"

Using typical parallelism as key to show that the right option are grammatically identical or similar in structure, sound, meaning, or meter and so on.

The answer A showed parallelism by having a structure that tactically showed "not only" and "but also" in the sentence.

Keyword 'broaden" is the verb original form in the context and was added behind "not only", in the options the verb original form show was added after "but" .

So Option B, Option C, Option D, and Option E can be excluded.

We see that a "not only" plus "but" is needed to ensure that the two phrases are connected in parallel