This sentence stands for the imperative mood. This mood is used when telling someone to do something. In other words, these are <em>commands. </em>So almuerce is the formal command that comes from the verb almorzar (to have lunch). Therefore, we can write and translate this sentence as follows:
<em>Señora, </em><em>almuerce </em><em>en el parque (Lady, have lunch in the park)</em>
Chica is feminine... reservada is feminine... so which of your options are feminine? Ella and la. Ella means she so it can't work, as that would mean she girl is very reserved. La is the femenine the. So that is your answer.
