Option 1 -> Colloids are indeed heterogeneous mixtures where particles are dispersed but not dissolved - this is a TRUE property.
Option 2 -> Colloids are STABLE systems where particles do NOT settle down when left undisturbed; this describes suspensions, not colloids - this is FALSE.
Option 3 -> Colloidal particles (1-1000 nm) are too small to be seen individually with naked eyes - this is a TRUE property.
Option 4 -> This describes the Tyndall effect, a characteristic property of colloids where particles scatter light - this is TRUE.
Hence, Option 2 -> The stability of colloids is what distinguishes them from suspensions. Unlike suspensions where particles settle due to gravity, colloidal particles remain dispersed indefinitely due to their size and charge interactions, making colloids stable systems -> correct