The best description of the market structure that Ashlee is entering is monopolistic competition.
Monopolistic competition is characterized by many sellers (in this case, food trucks and fast food restaurants), product differentiation (healthy alternatives to fast food), and relatively easy entry and exit from the market (which applies to starting a food truck). Ashlee's focus on healthy food options helps differentiate her business from both other food trucks and fast food restaurants, which is a key characteristic of monopolistic competition.