Correct Option: D

The adjective ‘tantamount’ which refers to ‘being essentially equal to something’ is generally followed by the preposition ‘to’ and not by ‘of’ or ‘with’. The correct expression is ‘tantamount to’.  This immediately eliminates options A and B.

Ex. His statement was tantamount to an admission of guilt.

Option C can be eliminated as well because the English language doesn’t have a word ‘tantamountly’.

Option D is the best choice among the available ones as in it the expression ‘tantamount to’ is followed by the gerund and thus forms a meaningful statement.

Option D is hence the correct answer.