A 23-year-old female presented with jaundice and pallor for 2 months. Her peripheral blood smear shows the presence of spherocytes. The most relevant investigation to arrive at a diagnosis is -
Young female, with 2 years history of anemia and peripheral blood demonstrating spherocytes points towards the diagnosis of auto-immune hemolytic anemia.
Immune hemolytic (autoimmune hemolytic) anemia is the most common cause of spherocytes.
It is caused by auto antibodies against RBCs.
Detection of autoantibodies by Coombs test will establish the diagnosis.

About option b

Spherocytes are also seen in hereditary spherocytosis, in which diagnosis is established by osmotic fragility test.
However, hereditary spherocytosis is usually presents in childhood or infancy.
Whereas as autoimmune hemolytic anemia is common in young females and it is the most common cause of spherocytes in the blood.
Reticulocyte count
Osmotic fragility test
Coombs test
Tests for PNH