A15 foot flagpole casts an 11 foot shadow. at the exact same time a 28 foot tree casts a shadow. which proportion would correctly find the length of the tree's shadow?

The longer an object, the longer the shadow they cast at a given time of the day and vice versa. This is called direct proportionality. At the same time,

If 15 foot flagpole casts 11 foot shandow
    28 foot tree casts ?? foot shadow

The proportion would therefore be:

Length of shadow cast by tree = (28/15)*11 = 20.533 foot.