The letter "R" is the symbol for:

In electrical and electronic formulas, such as Ohm's Law (E = I x R), the letter 'R' stands for resistance. Resistance is measured in Ohms and represents the opposition to the flow of electric current in a circuit. The other options have different standard algebraic symbols. Reactance is represented by the letter 'X', and impedance is represented by the letter 'Z'. Reluctance is a magnetic property, not typically used in basic electrical circuit formulas.