Learn Spanish (Latin America): ¿Estás seguro de la respuesta? Lo estoy.

In Spanish, when you reply to a question using estar or ser and want to say "I am" or "it is," you can't just leave the verb hanging. You must use the neuter pronoun lo to replace the adjective or state you are referring back to (in this case, seguro). It essentially means "I am it."

Why use de here instead of sobre for "about"?

The adjective seguro (meaning "certain" or "sure") is followed by the preposition de, not sobre. It is a set phrase: estar seguro de algo translates to "to be sure about something."

Could I use Eres instead of Estás here?

No, because using ser changes the meaning of the adjective. Estar seguro means to be "certain" or "sure" (a state of mind). If you said Eres seguro, it would mean "You are a safe person" or "You are reliable" (an innate characteristic).

If a woman is speaking, does Lo estoy change to La estoy?

No, it remains Lo estoy. Even though a female speaker would be segura, the pronoun replacing the adjective is the neuter lo. It never changes to match gender or number when it acts as a generic replacement for an abstract idea or state.