An example of relative time is:

a. october 12, 1492
b. the presidency of william henry harrison
c. the day before yesterday
d. february 12, 1906

An example of relative time is the day before yesterday.

Option C

Explanation:

An absolute time gives us a point of reference. It is also called chronometric time. Example of an absolute time includes saying 50 million years.  It is expressed in months, days,years, etc.  

A relative time is also called as chronostratic time. This type of time is not absolute but it is relative to other events.  

In the given options February 12, 1906, the presidency of William Henry Harrison and October 12, 1492 are absolute in nature.  

The option the day before yesterday is only relative. It is relative to a day before a previous day. Hence this option is an example of a relative time.