(b) Sodium hydroxide is a base which is soluble in water while Ferric hydroxide is also a base but it is not soluble in water.
The textbook (Chapter 2, Section 2.2.1) explicitly states: "Bases which are soluble in water are called alkalis." So the defining criterion for an alkali is water-solubility, not strength. NaOH dissolves in water → alkali; Fe(OH)₃ does not dissolve in water → base but not an alkali. Option (a) is partially correct but solubility, not strength, is the textbook definition. Option (d) is wrong in reasoning — "comparatively higher solubility" misses the point.