Solitary hypoechoic lesion of the liver without septa or debris is most likely to be -
A solitary hypoechoic lesion without any septae or debris is most likely to be a simple cyst. However rarely a hydatid cyst can have a similar sonologic appearance.
Simple cysts are the most common benign lesions found in liver. On imaging studies they appear as unilocular, homogenous fluid-filled structures with a thin wall without projections, septa or debris.

Ultrasound appearances of -
Hydatid cyst -
It presents either as a simple cyst with hydatid sand or daughter cysts or floating endocyst or calcification.
Caroli's disease -
Its the type V choledochal cyst
Multiple intrahepatic biliary cysts are seen on ultrasound. Sludge & stones may also be visible within the cysts.
Liver abscess -
Sonographic appearance may include a cystic lesion with floating internal echoes and irregular margins.
Septations, debris and fluid-fluid interfaces may also be seen.
Hydatid cyst
Carolis disease
Liver abscess
Simple cyst