perhaps the question is obvious but worded poorly or maybe it's to catch you out, the obvious answer is analogue, this is not digital input but converted to a digital representation of the analogue level or so it seems.
The problem is the wording, "prior to to input", an analogue is not converted to digital prior to input it is the input that is analogue, after input it is converted.
C&D I think are pretty obvious, A is defined as tiny magnetic fields stored digitally as north or south poles, so this is a bit of a conumdrum, although generally assumed to be digital it could be interpreted as converted from fields into digital ones & noughts, however, that is not input to a PLC or digital control board. so again the word "prior" could be confusing.
We know a disk is read in as a stream of digital data, as are switches or contacts, however, analogues are not converted to the input indeed they are converted after input into a digital representation (that is if we are talking about the Actual input).
This brings us back to Peter Nachtway's post earlier, poor or wrongly worded information given for others to give relevant answers.
Very often in exams what the original author wrote seems to them accurate & meaningful, however, as we all know often leads to mis-interpretation.
I'm sure that most of us know some questions are deliberately made to check that the participant is reading the question fully before answering.
An example is:
"Which inputs to a PLC are not digital", (Tick all that are relevant) whereas,
"Which inputs to a PLC are digital".