💡 Detailed Explanation
The correct choice is 'する' (dictionary form of 'する' - to do). The grammar pattern '〜ことにする' (to decide to make it a rule to do...) requires the dictionary form (or negative plain form '〜ない') of a verb. Sentence translation: 'I have decided to jog every morning starting next month.' The other choices ('します', 'して', 'した') are incorrect grammatical forms to precede 'ことにしました'.