Final answer:
The most suitable word to describe the witness who apologized is 'regretful', as it conveys the sense of remorse over wasting the police's time.
Step-by-step explanation:
The most appropriate word to describe the witness who apologized to police for wasting their time is regretful. This word suggests that the witness feels sorry for their actions and acknowledges the inconvenience that their mistake has caused. Mistaken could also apply but does not capture the element of apology or remorse implied in the statement, sincere indicates truthfulness and honesty which may or may not be associated with an apology, and ignorant generally refers to a lack of knowledge or awareness, which is not indicated in the statement about the apology.