The author believes that the extension of London's Ulez will ( ).

参考解析：

细节题。根据题干关键词extension和London's Ulez定位到第四段：And if Sadiq Khan wins and extends it to the North and South Circular roads in 2021 as he intends, it is sure to spark intense opposition from the far larger number of motorists who will then be affected.如果 Sadiq Khan在2021年胜出，并按照他的意愿将其延伸至南北环路，肯定会引发更多驾车者的强烈反对，届时他们将受到影响。选项B的ensure Khans electoral success确保Khans选举成功，文中提到的是如果这个人赢了，属于偷换概念。选项C的improve the city's traffic改善城市交通，定位段并未提到，属于无中生有。选项D的discourage car manufacturing抑制汽车制造业也并未在定位段提到，属于无中生有。故本题正确答案选A。