Step 1: Visualize the Problem.
The ladder forms a right triangle with the wall and the ground. The distance from the foot of the ladder to the wall is \( 2.5 \, \text{m} \), and the height reached by the ladder is \( 6 \, \text{m} \). The length of the ladder is the hypotenuse.
Step 2: Apply the Pythagorean Theorem.
\[
\text{Length}^2 = (2.5)^2 + 6^2 = 6.25 + 36 = 42.25.
\]
Taking the square root:
\[
\text{Length} = \sqrt{42.25} = 6.5 \, \text{m}.
\]
Step 3: Analyze the Options.
Option (1): \( 3.5 \, \text{m} \) — Incorrect.
Option (2): \( 4.5 \, \text{m} \) — Incorrect.
Option (3): \( 8.5 \, \text{m} \) — Incorrect.
Option (4): \( 6.5 \, \text{m} \) — Correct.
Step 4: Final Answer.
\[
\boxed{6.5 \, \text{m}}
\]