Arrange in increasing order of reactivity towards HCN: Acetone, Acetaldehyde, Di-tert-butyl ketone.
Reactivity decreases with steric hindrance. Di-tert-butyl ketone (bulky) < Acetone (less hindered) < Acetaldehyde (least hindered, aldehyde more reactive).
Ref: NCERT Class 12 Chemistry > Chapter 12: Aldehydes, Ketones and Carboxylic Acids > Topic: Nomenclature, Structure and Preparation of Aldehydes and Ketones