Only (B) and (E) survive — option 4 — and the other three each invert or garble a standard result.

(A) is wrong on two counts. The inverting summer's output is

\(V_{o}=-R_{F}\left(\dfrac{V_{A}}{R_{A}}+\dfrac{V_{B}}{R_{B}}+\dfrac{V_{C}}{R_{C}}\right)\)

First, an inverting configuration must produce a minus sign, and the statement has none. Second, its factor \(\left(1+R_{F}/R_{A}R_{B}R_{C}\right)\) is dimensionally impossible — adding 1 to a resistance divided by the product of three resistances gives units of reciprocal ohms squared. The correct expression simply multiplies the sum of input currents by \(R_{F}\), since all three currents meet at the virtual earth.

(B) is correct. The difference amplifier with matched resistors gives

\(V_{o}=\dfrac{R_{F}}{R_{1}}\left(V_{+}-V_{-}\right)\)

and with unity gain that is exactly the non-inverting input minus the inverting input, as stated.

(C) confuses two opposite filters. A notch filter is a narrow band-reject filter: it removes a narrow band and passes everything else, which is how mains hum at 50 Hz is eliminated. A narrow band-pass filter does the reverse. The two have complementary responses, so calling one by the other's name is a clear error.

(D) is backwards. VCO stands for voltage-controlled oscillator — a voltage in, a frequency out, so it is a voltage-to-frequency converter. The frequency-to-voltage converter is the opposite device, as Q13 of this paper describes.

(E) is correct, and describes exactly what an all-pass filter is for. Its magnitude response is flat at unity for every frequency:

\(H(s)=\dfrac{s-a}{s+a},\qquad \left|H(j\omega)\right|=1\)

while its phase varies from 0° to −180°. Since it changes nothing about amplitude, its whole purpose is phase — delay equalisation in transmission lines, phase shifters in SSB modulators, and correcting the group delay of other filters.

Hence, the correct statements are (B) and (E).