It's becoming increasingly difficult to control the growing mob culture in the…

The idiom 'take a toll on something' means to cause damage, harm, or a negative effect to something that holds value — it describes an existing positive or valuable state being worn down or eroded.

Here, the growing mob culture is described as difficult to control and as 'taking a toll' — so the blank must name something valuable that this negative force is eroding. An otherwise good image of the city fits: it names a reputation that was positive before the unrest began damaging it.

• a poor image of the city: an image already described as poor carries no established value left for the mob culture to erode, so it does not fit the damage/erosion sense of the idiom.

• a neutral image of the city: a neutral image has no positive standing to be worn down, so it does not match the harm implied by 'taking a toll.'

• an ever changing image of the city: this describes ongoing variability, not damage or erosion, so it does not capture the negative effect the idiom describes.

Only an otherwise good image names a valuable state that the mob culture's toll can erode, which is why that option completes the sentence correctly.