A. This one of the seven rabbinical mitzvot, introduced by the sages in the spirit of the Torah. Their authority is based on Deut. 17:8-11 which tells every generation to heed “the judge that shall be in those days”.
Other mitzvot of the sages include kindling Shabbat lights, reciting Hallel and reading the Megillah. Add the seven rabbinical mitzvot to the 613 written commandments and you get 620, which is the numerical value of keter – “crown” (of the Torah).