Sir Thomas Brown’s longest work, Vulgar Errors, is about-

The correct option is: Vulgar Beliefs and practices of the time.

Sir Thomas Browne’s longest work is officially titled Pseudodoxia Epidemica, or, Enquiries into Very many Received Tenents, and commonly Presumed Truths (first published in 1646), but it is widely known as Vulgar Errors.
In the context of the 17th century, the word "vulgar" did not mean offensive or crude; rather, it derived from the Latin word vulgus, which means "the common people" or "ordinary public." Thus, "vulgar errors" translates to "common or popular misconceptions."

As a physician, philosopher, and polymath, Browne wrote this monumental treatise to systematically examine, critique, and debunk the widespread superstitions, scientific errors, mistaken folklore, and unverified beliefs of his era.
Throughout the work, Browne tackles false opinions concerning various subjects, including mineral and vegetable bodies, animals, humans, geography, history, and biblical interpretations, promoting empirical observation and critical inquiry instead of blind adherence to tradition.