The correct answer is Option D.
We are to complete the given sentence by using the nominalisation of the verb 'dismissed'.
Nominalisation is the noun form of a verb or an adjective.
Options A and B, DISMISS and DISMISSING are both verb forms.
Option C: DISMISSION is the noun form but exactly a correct word to use.
Hence these options are incorrect.
Option D 'DISMISSAL' is the correct noun form of the form and also satisfies the meaning of the given sentence.