In a Spanish conversation, the suitable response to 'Mucho gusto' is 'El gusto es mío' or 'Encantado/a'.
The correct missing word is 'a. Encantada' based on the conversation provided.
In Spanish, when someone says 'Mucho gusto', a suitable response would be 'El gusto es mío' or 'Encantado/a' to express pleasure in meeting the person.
Therefore, in this context, 'a. Encantada' is the appropriate choice as it aligns with the polite exchange of greetings.