Final answer:
The mode of administration that is most susceptible to missing responses is self-selected samples.
Step-by-step explanation:
The mode of administration that is MOST susceptible to missing responses is self-selected samples. When surveys are conducted and responses are only obtained from people who choose to respond, such as in internet surveys, the results can often be unreliable. This is because the sample may not accurately represent the target population, leading to biased or incomplete data.