The correct option is C Polygamy
Option C 'Polygamy' is the custom of having many wives. Option A 'Monogamy' is practice of marrying one person at a time. Option B 'Bigamy' is offence of marrying someone who is already married to another person. Option D 'Matrimony' is marriage or wedlock. Thus, options A, B and D are incorrect and the correct answer is option C, Polygamy.