@hotpaw2 It's not an assumption, it's the definition of measurement. Superposition is not a state you can observe. See also : Wave function collapse.

@hotpaw2 Yes, but don't confuse that with an electron in an operating, engineered device where it is forced by the environment to assume a given state. That non-zero probability only applies to electrons before they are measured. Upon measurement there can still only be two states - electron detected or electron not-detected. That's fundamentally a binary system. What you're talking about would fall under the umbrella of the device's reliability or repeatability (ie: does a "stored" electron stay or not). It doesn't change the discrete nature of the options available.

The opening sentence is misleading. "Digital" means "discrete", most generally, and there certainly are discrete phenomena in the physical world. The entire world of quantum mechanics opens the possibility for many types of quantized electronic systems which can operate on single electrons, etc. There is no fundamental "analog" nature to such a system - either an electron is present or it is not, so the device always exists in one of two available states.

@CrisLuengo No, I worded it awkwardly. The point was that with multiple choice you don't get to see the thought process of the student and this is a question which lends itself particularly well to a bit of exposition.