Cleft lip results from the failure of which fetal tissue to fuse?

Cleft lip occurs due to the failure of the left and right sides of the fetal lip tissue to fuse properly during early fetal development. This fusion typically happens between the sixth and eighth weeks of gestation. When this normal process is disrupted, it results in a cleft lip, which can vary in severity from a small notch in the lip to a complete separation that extends up into the nasal passages.

Focusing on the other options, the tongue and cheeks are not directly involved in the formation of the cleft lip. While the nasal passages can be affected by cleft lip condition, they do not play a role in the fusion of lip tissue itself. Therefore, the correct answer highlights the specific tissues involved in the development of a cleft lip, providing the clarity needed to understand this congenital condition.