The inguinal ligament spans from the $\text{ASIS}$ to the $\text{pubic tubercle}$ and marks the boundary between the abdominal wall above and the thigh below. The question is essentially asking which flat abdominal muscle gives rise to it.
Of the three flat muscles of the anterolateral abdominal wall, the $\textbf{external oblique}$ is the most superficial. Its aponeurosis, on reaching the lower margin, folds backward on itself like a hem; this in-rolled lower border IS the inguinal ligament. Hence it is a direct modification of the external oblique aponeurosis.
The internal oblique and transversus abdominis instead arch over the spermatic cord and unite medially as the conjoint tendon, while the internal oblique also supplies the cremaster muscle. The rectus abdominis is a vertical strap muscle that takes no part in forming this ligament. Several secondary ligaments - lacunar, pectineal and reflected inguinal - are likewise extensions of the same external oblique aponeurosis.
\[\boxed{\text{Inguinal ligament = modified lower border of external oblique aponeurosis}}\]