Four identical resistors, each of resistance R, are connected only in series and/or parallel, with every resistor used and none bypassed. Which equivalent resistance cannot be obtained? Treat 1.33R as the rounded value of 4R/3.

Explanation

With four identical positive resistors restricted to series/parallel connections and all four used, 2R is not obtainable. The other choices are R/4 (all parallel), 4R/3 (one in series with three parallel), and 5R/2 (two in series plus a parallel pair). The printed 1.33R is a rounded value of 4R/3.