What do we call a simple word that can become a new word by having more letters added to it?

The correct choice is a root word. A root word serves as the foundation of a word, and by adding prefixes or suffixes (additional letters or groups of letters), it can transform into a new word with a different meaning. For example, "play" is a root word that can become "playing," "replay," or "playful."

Understanding root words is fundamental in language, as it helps in recognizing and deciphering unfamiliar words by identifying their component parts. This concept is crucial for vocabulary development and enhances reading comprehension skills.

In contrast, base words also refer to similar concepts, but the term "root word" is often more specifically used in linguistic contexts to describe the simplest form of a word. Understanding affixes is important, but they themselves are not words; they are additions made to a root or base word. Compound words are formed by combining two complete words to create a new meaning, which is different from adding letters to a simple word.