Which winter annual broadleaf weed has a tomentose rosette with square stems and yields bright yellow flowers in clusters?

The winter annual broadleaf weed characterized by a tomentose rosette with square stems and bright yellow flowers in clusters is Black Medic. This plant, scientifically known as Medicago lupulina, is identifiable by its unique growth habit and leaves, which have a distinctive hairy or 'tomentose' texture. Additionally, Black Medic produces small bright yellow flowers grouped in clusters, which makes it recognizable during flowering periods.

The other options, while also representing different types of broadleaf weeds, do not share the same combination of features. For example, Carolina Geranium produces pink to purplish flowers and lacks the square stem characteristic. False Dandelion, although it has a yellow flower similar to dandelions, differs significantly in terms of leaf structure and growing habits. Dandelions, on the other hand, feature a deep taproot and a rosette of leaves that differ from those of Black Medic, primarily in leaf morphology and stem structure. Thus, Black Medic stands out due to its unique combination of characteristics.