Final answer:
The absence of a control group (d) is an internal validity problem when a design lacks a comparison method. Control groups are necessary for comparing outcomes and establishing causal relationships, as they ensure that differences observed are due to the variable being tested rather than extraneous factors.
Step-by-step explanation:
The internal validity problem with a design that lacks a control or comparison method is known as the Absence of control group (d). A control group is crucial because it serves as a benchmark to compare the outcomes of the experimental group against. Without this comparison, it's challenging to determine if the changes observed in the experimental group are due to the independent variable or other extraneous factors. Experimental controls are vital as they allow comparison between groups that are different in only one independent variable. This focused difference is essential for establishing causal relationships in research. For example, if you're testing the effectiveness of a new teaching method on student performance, your control group would use the standard method, while the experimental group would use the new method. Both groups should be identical in every other respect, to ensure that any differences in performance are due to the teaching method itself and not other variables. This methodical approach ensures the integrity of the experiment and helps to minimize the influence of confounding variables.