Answer:
Option A, After walking to school, the bus slowly passed by.
Step-by-step explanation:
When the sentence itself is not clear about what is being modified, then the modifier is termed as dangling modifier
Sometime these dangling modifier refer to a word with in a sentence that does not even appear in it.
Here the phrase "After walking to" does not make any sense as a modifier . it is not directly modifying the work school but it intends to modify it.
Option A is correct