The correct answer is C. Time.
A strip chart recorder is a type of data logger that records data as a function of time. It does this by using a self-balancing potentiometer to convert the input signal into a voltage, which is then used to move a pen across a moving chart. The pen traces out a line on the chart, which represents the data as a function of time.
The other options are incorrect because they are not the primary function of a strip chart recorder. Option A, another emf, is not the primary function of a strip chart recorder. The primary function of a strip chart recorder is to record data as a function of time. Option B, frequency, is not the primary function of a strip chart recorder. The primary function of a strip chart recorder is to record data as a function of time. Option D, pressure, is not the primary function of a strip chart recorder. The primary function of a strip chart recorder is to record data as a function of time.