Answer:
The sentence with the best elements of parallelism of the examples provided is Option B. Science may be challenging, but it is also rewarding; reading might be time-consuming, but it is also enlightening.
Step-by-step explanation:
Parallelism is a property of the way your sentences are structured in a piece of writing. It is usually a property that is particularly important when providing lists in something you write. All points on a list should start with a word in the same format, i.e. the listed elements should all start with a noun -- or a verb, if that is the case -- but not a mixture of those two elements. You see this mistake a lot on peoples resumes when they are listing things. Option B is best because "reading" is treated like the noun "science" in this case. Another example of the work "reading" being treated like a noun would be: "I have done the reading." Both sentences use the verb in the present progressive form in the second independent clause ending each sentence. That is another way they are parallel. There are other grammatical similarities that could be considered parallelsim in this example too!