(a) To find the probability that a tree is classified as short, we calculate:

\(P(X < 25) = P\left( Z < \frac{25 - 40}{12} \right) = P(Z < -1.25)\)

Using the standard normal distribution table, \(P(Z < -1.25) = 0.106\).

Thus, the probability is 0.106.

(b) Given that one third of the trees classified as tall or medium are tall, we have:

\(P(\text{tall}) = \frac{1}{3} \times (1 - 0.106) = \frac{0.8944}{3} = 0.298\).

Therefore, the probability that a randomly chosen tree is classified as tall is 0.298.

(c) To find the height above which trees are classified as tall, we use the probability found in part (b):

\(0.2981\) gives \(z = 0.53\).

Using the standard normal distribution formula:

\(\frac{h - 40}{12} = 0.53\)

Solving for \(h\):

\(h = 0.53 \times 12 + 40 = 46.4\).

Thus, trees are classified as tall if their height is above 46.4 m.