The 'x-point' in the context of the x-wind model refers to the location where the accreting material meets the star's magnetosphere and the gas is launched outward to become the x-wind. This model was proposed by Shu et al. (1988) to explain the formation of protoplanetary disks and the process of planetesimal accretion. The x-wind is a bipolar outflow of gas from the inner region of the disk, which carries away angular momentum and allows material to accrete onto the star. The outer edge of the protoplanetary disk: This is not the correct definition of the x-point. The x-wind model deals with the inner region of the disk, not the outer edge. Where the photosphere of the star meets with the accreting material: While this location is important for understanding the accretion process, it does not correspond to the x-point in the x-wind model. Where the mantle and the core meet within a differentiated planetesimal: This definition is unrelated to the x-wind model, which focuses on the interaction between the star's magnetosphere and the protoplanetary disk. Final answer: The x-point is where the accreting material meets the star's magnetosphere and the gas is launched outward to become the x