According to general rule, the possessive of a singular noun (here, 'river') is formed by adding an apostrophe (') and s. Therefore, the correct form of writing the possessive of river is 'river's'. Only and apostrophe is added for nouns that are plural and end with an s.Therefore the correct form of writing the possessive of boys is boys'.
Thus, by applying the rules of using apostrophe and eliminating the incorrect options, the correct answer must be Option C.