What's the question?
Out of the options, what does the sentence 'I saw a woma walking across the bridge' tell you?
That you saw a woman walking across the bridge or that you saw a woman as you were walking across the bridge.
The second option given in the poll is correct. (I think :-x please don't surprise me people)
To me it is ambiguous
If it just heard "I saw a woman walking across the bridge" I would assume she was walking across the bridge but when you think about it it can also mean the other
Id assume if someone said this to me there would be more to the conversation and the way it was meant would become apparent
The sentence is ambiguous.
1. [I saw] [a woman walking across the bridge]
2. [I saw [a woman] [(while) walking across the bridge]
Admittedly, solution 1. above should be the most naturally inituitive, as the other has a more complex structure. The mind likes to parse (break down) language into the simplest structures it can. But both are possible.
It is ambiguous. However, the second option seems more likely to be correct, since for the first meaning, I would choose a different word order - Walking across the bridge, I saw a woman.