The last option is correct. Fui al mall y compré un vestido azul.
Explanation: It was 8pm. My sister’s birthday was next day and I needed to buy a gift for her, so I went to the mall and I bought a blue dress (fui al mall y compre un vestido azul).
Use el when taslking about a guy and ella when talking about a girl. so here is an example El grande
