The answer is: A. 20.3
The explanation is shown below:
1. By definition, the sum of the length of the strings (The length of the blue segment and the length of the red segment) is equal to the length of the major axis (The black segment).
2. Based on this, you can write the following expression, where is the length of the blue segment:
3. Now, you must solve for , as following:
4. Therefore, the answer is the option A.