The correct option is C barren
Fecund means producing or capable of producing an abundance of offspring or new growth; highly fertile. The opposite of fecund is barren, or (of land) too poor to produce much or any vegetation. Hence, Option C is correct. Among the rest of the options, fertile means (of soil or land) producing or capable of producing abundant vegetation or crops, obsolete means not relevant anymore, fruitless means failing to achieve the desired results. These options do not express this antonym, hence incorrect.