Very briefly answer following $5$ small questions:

a$,$ In OP$-$Amp, Resistor R f is used for what purpose?

b$,$ Binary numbers $10111001+10111011=?$ for Binary and then for Decimal

c$,$ Conversion following Decimal numbers $(121)$ to Binary numbers

d$,$ in digital electronics, prove A$+$B$=$A$+$B