What is another word for the act of making a decision?

How about committed?

Meaning - to pledge (oneself) to a position on an issue or question; express (one's intention, feeling, etc.):

Asked if he was a candidate, he refused to commit himself.

The ability to do so is decisiveness (noun) and such a person can be called decisive (adjective).

ODO:

decisiveness NOUN

1 The ability to make decisions quickly and effectively.
‘the crisis will require leadership and decisiveness to resolve it’

decisive ADJECTIVE

1.1 (of a person) having or showing the ability to make decisions quickly and effectively.

‘Like Clifford, he is decisive and very skilled at pushing agendas through.’