The correct answer is A. oscillations.
Regenerative feedback is a type of feedback in which the output of a system is fed back into the input in such a way that it causes the system to oscillate. This can be useful in some cases, such as in oscillators, but it can also be problematic in other cases, such as in amplifiers.
A step input is a type of input signal that changes abruptly from one level to another. This can be useful in some cases, such as in testing the response of a system to a sudden change in input, but it can also be problematic in other cases, such as in systems that are designed to operate continuously.
A negative sign in feedback implies that the output of the system is subtracted from the input. This can be useful in some cases, such as in amplifiers, but it can also be problematic in other cases, such as in oscillators.
A positive sign in feedback implies that the output of the system is added to the input. This can be useful in some cases, such as in oscillators, but it can also be problematic in other cases, such as in amplifiers.
In conclusion, the correct answer is A. oscillations.