【正确答案】 E

【答案解析】Idiom; Verb form
The sentence explains that fish discharge electric currents for several purposes, which are most efficiently and effectively described in a parallel structure: to find and attack, to defend, or to communicate and navigate. The use of either is inappropriate in this sentence because more than two uses of electric currents are listed; idiomatic usage requires either to be followed by or to identify alternatives, not by also.
A Either inappropriately introduces a list of more than two alternatives, and it should not be followed by or also; parallelism requires that for communicating and navigating be changed to to communicate and navigate.
B Defend is not parallel with the list of gerunds, leaving the reader to wonder how to make sense of defend themselves.
C The lack of parallelism obscures the relationships among the items in the series; it is especially confusing to list an infinitive phrase (to find...), an object of a preposition (for defense), and nouns with no grammatical connection to the verb phrase (communication and navigation).
D This answer choice also violates parallelism by mixing an infinitive with objects of the preposition for. Or also is an unidiomatic, contradictory expression.
E Correct. The different ways in which the various species offish use their electric power are correctly expressed in a series of parallel infinitives.
The correct answer is E.