The sentence that incorporates parallel structure is:
D. During my week at camp, I scaled the climbing wall, built a fort and jumped on the trampoline.
All the verbs in that sentence are in simple past tense (scaled, built, jumped).
<u>Why not the others?</u>
A. My week at camp included rock-climbing, mountain biking and sleep. In order to have a parallel structure the statement should have said <em>sleeping </em>instead of<em> sleep.</em>
B. During my week at camp, I swam, hiked, biked and skiing on the lake. In order to have a parallel structure the statement should have said <em>skied </em>instead of<em> skiing.</em>
C. My week at camp consisted of playing soccer, riding mountain bikes and crafts. In order to have a parallel structure the statement should have said <em>making crafts </em>instead of<em> </em>just <em>crafts.</em>