When an input electrical signal A=10100 is applied to a NOT gate, its output signal is

The correct answer is $\boxed{\text{D. }101}$.

A NOT gate is a logic gate that inverts its input signal. This means that if the input signal is 1, the output signal will be 0, and vice versa.

In this case, the input signal is $A=10100$. This can be represented as a binary

number, which is $16$ in decimal. The output signal of the NOT gate is therefore $101$, which is $13$ in decimal.

Option A is incorrect because it is

the complement of the output signal. Option B is incorrect because it is the complement of the input signal. Option C is incorrect because it is the complement of the output signal plus 1. Option E is incorrect because it is not a valid binary number.