First of all, you need to know what a dangling modifier is. It means that there is a modifier which refers to the wrong thing in a sentence. Here, that would be 'digging to the bottom of my suitcase.' As you can see, the way this sentence is written, it would seem that modifier refers to 'my passport' as if it was digging, which is not true.
Therefore, the correct answer which fixes this problem is A. Digging to the bottom of my suitcase, I failed to locate my passport, which was hidden under a pile of dirty clothes
