Zachariah has been asked to assess whether the backup process at his organization is sufficient to meet all of the company's goals and objectives, which support their key performance indicators. One of the topics that he is analyzing is the frequency of backups to ensure that the company does not lose any more than $6$ hours of data in the case of a hard drive or other system failure. Which of the following describes the particular metric that he is concerned about?

a$.$ RPO

b$.$ RTO

c$.$ MTTR

d$.$ MTBF