The company improving its service is the statement that uses the pronoun-antecedent agreement.
Option (b);
EXPLANATION:
Antecedent is the noun that precedes the pronoun it refers to. The main rule of identifying an antecedent is that if the antecedent is plural, the pronoun should also be plural.
All the sentences except sentence B are grammatically incorrect. Sentence A refers to one of the girls, so it should be her speech and not their speech.
In the sentence, C shows that everyone wears their costumes. In sentence D, seeds are plural, so it should soak them in a blue food coloring.