Answer:
"Solía cantar"
Step-by-step explanation:
The setence "I used to sing", it's refering to an activity in the past, the subject sang before, but not know, this can be expressed in Spanish as:
"Solía cantar"
If you wanna use the subject would be :"Yo solía cantar".
These statements indicates the same intention as the given sentence.
Therefore, the best answer here is "Solía cantar".