毎朝、公園___走っています。
Q. Which particle marks the park as the space moved through?
Translation
I run through the park every morning.
Explanation
Motion verbs like 走る, 歩く, 飛ぶ take を for the space traversed: 公園を走る 'run through the park'. 公園で走る is also grammatical but frames the park merely as a location; the 'path' reading with を is the standard pattern being tested.