Select the option that can be used as a one-word substitute for the underlined group of words.

She is proficient in speaking many languages.

The correct option is multilingual.

Let's analyze the meaning of the underlined phrase and the given options to find the appropriate one-word substitute:
The phrase "proficient in speaking many languages" refers to someone who has the ability to use or speak several different languages.

Now, let's look at the definitions of the options:
1. Multilingual: Able to speak, write, or use several (many) languages. The prefix "multi-" means "many" and "-lingual" relates to tongue or language. Thus, it is the perfect substitute.
2. Bilingual: Able to speak two languages fluently. The prefix "bi-" means "two".
3. Heterolinguistic: Relating to or characterized by different languages; however, it is not a standard one-word term used to describe a person who speaks many languages.
4. Monolithic: Literally meaning formed of a single large block of stone, or metaphorically describing a system that is massive, solid, and uniform. It is unrelated to speaking languages.

Therefore, multilingual is the correct one-word substitute for "proficient in speaking many languages".