Read the question. All of the clues are there.
1) The manufacturer says the cell produces 1.5V e.m.f. and 100mA current.
2) Use ohms law to calculate a suitable load resistor which will draw 100mA from the cell at that voltage.
3) Draw a circuit schematic showing both the load resistor and the cell with a resistor representing it's internal resistance.
3) Place the load resistor across the cell terminals.
4) Place a voltmeter across the load and an ammeter in series with the load.
5) Measure the voltage and current and plot a graph.
6) There will be a discrepancy. At the calculated load resistance, the voltage across the load will be less than 1.5V and the current will also be less than 100mA because of that internal resistance.
So how would you go about working out what that internal resistance is?
What errors do you expect and what would you do to minimise them? CLUE: that variable resistor.