Which factor typically aids in the spread of noxious weeds?

Human activities significantly contribute to the spread of noxious weeds, making this the correct answer. Practices such as farming, landscaping, and construction often disturb the soil and alter natural habitats, creating favorable conditions for the introduction and proliferation of these invasive species. For example, tilling soil can bury native seeds while exposing new ground for weeds to take root. Additionally, equipment and machinery used in these activities can inadvertently transport weed seeds from one location to another, further facilitating their spread.

In contrast, limited access to water sources generally hinders the growth of plants, including noxious weeds, as many of them require adequate moisture to establish themselves and thrive. High levels of soil nutrients might support the growth of various plants, including noxious weeds, but it is the human-mediated disruptions that primarily facilitate their spread. Lastly, a decline in temperature rates is unlikely to aid in the proliferation of noxious weeds; lower temperatures often limit plant activity and may lead to reduced growth or survival of certain invasive species.