Which best describes eras and periods?

Eras and periods both appear at the start of the Cambrian period.

In geological time scale, period is the fundamental unit.

Few periods combine to form an Era.

Few Eras combine to form an Eon.

Time period before the Cambrian period (which is the first period in Paleozoic era) is generally termed as 'Precambrian' time. Classification of time scale as periods and eras began at the start of Cambrian period.

the answer to your question is b