Answer is (B) Line Graphs.
How X varies with Y, and what functional form it takes can be studied using a simple line graph.
Rationale:
Models are created to study the variables that are dependent on the independent variables. It is not a visual representation of relationship between the two dependent variables.
Pie charts are created to show the distribution of quantities in percentages. It does not highlight a relationship between two dependent variables.
Complex systems are used to model the systemic changes in a real-world scenario. It is not used to give a visual representation of the relationship between two dependent variables.