Normal size but non functioning uterus is usually associated with:
Lets have a look at each option:
Option a: Stenosis of external os would result in hematometra, i.e. uterus would be large and non functioning (hence ruled out).
Option c: Partial agenesis of vagina 
In parital agenesis of vagina a segment of vagina may be atretic in the upper-third. It is often associated with hypoplasia or even absence of cervix. Uterus may be normal and functioning or malformed.
Option d: Complete absence of vagina is almost always associated with absence of uterus.
Hence by exclusion our answer is b i.e. uterine synechiae (Asherman syndrome)
In Uterine syechiae–the uterus does not function normally due to presence of adhesions size of uterus is normal.
Stenosis of the external os
Uterine synechiae
Partial agenesis of the vagina
Complete absence of vagina