Final answer:
The C++ code in question manipulates a vector of test scores but requests the output of an element that has not been initialized or modified within the code provided. The default value of vector elements in C++ is not defined by the language specification without an explicit initializer; thus, the output cannot be determined from the given options.
Step-by-step explanation:
The provided code is in C++ which initializes a vector testScores with 8 elements. The loop fills the first six elements of the vector with the value 78.4. After the loop, i equals 6 and testScores.at(i) is set to 97.2. The index is then incremented to 7 and testScores.at(i) is set to 87.5. The index is decremented back to 6 and that position is then set to 91.4. The output is then requested for the element at the 8th position in the vector (index 7), which has not been modified since initialization. Thus, without knowing the default value of uninitialized elements in the vector, we cannot provide a specific output from the options given; the output is dependent on the initial state of the vector elements or the default value assigned during the creation of the vector, which is not specified in the given code.