Either
The overpopulation problem is so dire that selecting some humans to be a food source is a just solution.
Or
There are no reasonable objections to the author's proposal.
The use of a, an, or the is a signal that a noun is coming.
In the English language, only nouns are preceded by articles. A is used when a noun starts with a consonant (a book, a table, a bottle); an is used when the noun starts with a vowel (an apple, an umbrella, an avocado); the is used when you are referring to a particular thing (The boy I just met is called Mark.)
The answer is C. a stone monument
