Answer:
The correct answer is A. Louise will be swimming, biking and running for her triathlon this weekend.
Step-by-step explanation:
A parallel sentence is a sentence that uses the same form of a word throughout. In the sentence above, all verbs are used in their future progressive tense: will be swimming, (will be) biking, (will be) running. Therefore, their form is the same, and thus the sentence is parallel.
Sentence B is incorrect because different forms of the verbs are used: will be swimming, bike, run - as you can see, they're not the same.