B.On the hike, Mr. Clark tripped over a tree root, fell to the ground, and broke his wrist.
Parallel structure is using the same grammatical structure for a list of items or ideas. In option B, all of the actions in the list are written in simple past tense. They do not include a helping verb or other form. The verbs are all the same structure: tripped, fell, broke.
Option A is not correct. To make it correct parallel structure, "being responsible" should be "responsibility". To correct the parallel structure for Option C, "babysitting" should be "to babysit". To fix option D, add "having" before fierce determination.
