Answer: to avoid bias and to get a representative sample
Step-by-step explanation:
A random sample is useful for:
Avoid bias: if you have a selected sample, all of the selected individuals may have some common factor (same age group, same salary rage, same political ideology) so the sample may be kinda biased, this is something that you may want to avoid.
All the other options can not be ensured by any method, you survey, even with a random sample, may have no significant result, and you can not ensure that the surveyed people will answer truthfully.