**Solution:**

To solve this problem, we can use the equations of motion for a freely falling object. Let's break down the problem into steps:

**Step 1: Define the variables:**
- Height of the window, h = 2.05 m
- Time taken to cross the window, t = 0.1 s
- Acceleration due to gravity, g = 10 m/s^2

**Step 2: Determine the initial velocity:**
Since the stone is falling freely, its initial velocity is zero.

**Step 3: Calculate the final velocity:**
Using the equation of motion: v = u + gt (where v is the final velocity, u is the initial velocity, g is the acceleration due to gravity, and t is the time taken), we can substitute the values:
v = 0 + (10)(0.1) = 1 m/s

**Step 4: Find the distance traveled above the window:**
We need to find the height above the window that the stone has traveled in the given time.

Using the equation of motion: h = ut + (1/2)gt^2 (where h is the height, u is the initial velocity, t is the time taken, and g is the acceleration due to gravity), we can substitute the values:
2.05 = 0(0.1) + (1/2)(10)(0.1)^2
2.05 = 0 + (1/2)(10)(0.01)
2.05 = 0.05

Therefore, the height above the window that the stone has traveled is 0.05 m.

**Step 5: Calculate the total height above the window:**
To find the total height above the window, we add the height of the window to the height above the window that the stone has traveled:
Total height = h + height above window = 2.05 + 0.05 = 2.1 m

Therefore, the height above the window is 2.1 m.

**Step 6: Choose the correct option:**
Comparing the calculated height (2.1 m) to the given options:
a. 10m - Incorrect
b. 15m - Incorrect
c. 5m - Incorrect
d. 20m - Incorrect

None of the given options match the calculated height above the window of 2.1 m.

Hence, the correct answer is not provided in the options.

Note: Please ensure that you have provided the correct options for the answer.