Rashi: It is strong, like "Ofel va'Vachan", "Hekimu Vachonav" (Yeshayah 32:14, 23:13).

Radak: It is a fortress - "Hekimu Vachonav" (Yeshayah 23:13). Even though you [Yirmeyah] will say harsh words to them, do not fear them. Also at the beginning of his Nevu'ah, Hashem said "Nesaticha ha'Yom l'Ir Mivtzar" (1:18).

Radak citing Targum Yonasan: It is the choice (favorite).

Malbim: Discern. Yisrael are compared to impurities in a smelting pot of silver - copper, tin, iron and lead. There were many sects; each did its own evil, but not evil of the other sects. This one did adultery, this one murdered, this one did idolatry

Rashi: You will rebuke them, and they will not listen.

Radak: Seek their ways ¹ . It says v'Vachanta for a similar expression to Bachon.

Malbim: Refer to 6:27:1:4.