Final answer:
Taproot-anchored weeds are invasive and problematic for gardeners because they not only anchor to the soil strongly but can also harm native plants and ecosystems through toxic root exudates.
Therefore, option a is correct.
Step-by-step explanation:
Weeds with taproots can be problematic for gardeners because they are typically invasive. These plants with taproots are not easily uprooted by wind or animals due to the strong anchorage the taproot provides. Moreover, certain invasive species release root exudates that are toxic to native plant species and their symbiotic partners, such as arbuscular mycorrhizal fungi, which can lead to a decrease in native plant biodiversity. For instance, garlic mustard, an invasive species, releases root exudates that harm these beneficial fungi in North American soils.
Furthermore, some invasive species can also spread rapidly and replace native plants, leading to alterations in the ecosystem. Such invasive weeds are a challenge as they adapt to and dominate less disturbed habitats, outcompeting the native flora. Additionally, the definition of a weed is situation-specific; they are considered pests when they are undesirable in a particular scenario.