Solitary hypoechoic lesion of the liver without septate or debris is most likely to be-
A solitary hypoechoic lesion without any septae or debris is most likely to be a simple cyst. 

          Option b requires specific mention here:-

Solitary hypoechoic / anechoic lesion without septae or debris can occur in the early stage of evolution of hydatid cyst (Type 1 / Simple cyst). However, the diagnosis of hydatid cyst may be considered when focal thickening of the wall is present or when fine hyperechoic spots, due to the hydatid sand, appear in the dependent areas.

          Solitary hypoechoic aseptate lesion           → Simple liver cyst
          Solitary hypoechoic aseptate lesion with    → Type 1 (simple) hydatid cyst 
          hyperechoic spots due to hydatid sand
Hydatid cyst
Caroli's disease
Liver abscess
Simple cyst