Which type of tree growth habit is characterized by a narrow, upright form?

The growth habit characterized by a narrow, upright form is known as fastigiate. Fastigiate trees have branches that grow vertically and closely together, creating a columnar appearance. This specific growth pattern allows fastigiate trees to take up less lateral space, which can be a significant advantage in urban settings or small garden spaces where width is limited but height remains desirable. The vertical growth habit can also create an attractive feature in landscape design, drawing the eye upwards and adding elegance to the area.

The other growth habits mentioned differ in their structural characteristics. Pyramidal trees, while also having a pointed shape, typically have a broader base compared to fastigiate varieties. Weeping trees have a drooping or cascading growth form that does not lend itself to an upright structure. Broad trees tend to have a wide canopy and a more expansive growth habit, which contrasts with the narrow form exemplified by fastigiate trees. Thus, fastigiate is the correct response due to its distinct upright, narrow growth form.