Concept:The noun "people" is countable and plural. The missing word must be a quantifier that works correctly with countable plural nouns.
Explanation:Countable nouns are things we can count, such as people, books, or chairs. Uncountable nouns include water, milk, or money.
Use "many" with countable plural nouns. So we can correctly say: "We saw many people in the room."
Now check the other options.
"A little" is used only with uncountable nouns, like "a little milk." Therefore, "a little people" is wrong.
"Much" is also used only with uncountable nouns, like "much time." Therefore, "much people" is wrong.
"Plenty" must be followed by "of" before the noun, so the correct phrase is "plenty of people," not "plenty people." Hence, option C does not fit.
This leaves "many" as the only word that fits correctly in the sentence.
Answer:D. many
We saw many people in the room.