A person observes several trees in an open space, with the...

The apparent size of an object is proportional to the visual angle it subtends at the eye. For small angles, this angle is given by $\theta \approx \dfrac{h}{d}$, where $h$ is the height of the object and $d$ is its distance from the eye.

Calculating the visual angle for each tree:

For tree A: $\theta_A = \dfrac{2.0}{50} = 0.040$ rad

For tree B: $\theta_B = \dfrac{2.5}{80} = 0.03125$ rad

For tree C: $\theta_C = \dfrac{1.8}{70} \approx 0.0257$ rad

For tree D: $\theta_D = \dfrac{2.8}{100} = 0.028$ rad

Comparing the angles, we get $\theta_A > \theta_B > \theta_D > \theta_C$.

The decreasing order of their apparent sizes is A, B, D, C.