A ball thrown vertically upwards after reaching a maximum height h returns to the starting point after
a time of 10s. Its displacement after 5 s is   

The position of an object is always expressed w.r.t. some reference point. If the initial position of an object w.r.t. a reference points is s₁ and after sometime, it changes to s₂, then the magnitude of the displacement of the object is s₂ - s₁. Since, the given ball returns to its starting point in 10s. So, in 5s, it reaches the highest point which is at s₂.
Given, s₂ is at a height h w.r.t. s₁. Thus, the displacement after 5s will be s₂ - s₁ = h.