The correct answer is option 4, "Forest crops." Silviculture refers to the cultivation and management of forests and forest crops. It involves the science and practice of growing and tending to forest trees for various purposes like timber production, ecological restoration, and biodiversity conservation. Silviculture includes activities such as planting, pruning, thinning, and harvesting of trees.

Option 1, "Culture of Hilsa," is not related to silviculture. Hilsa is a type of fish, and its culture or cultivation would fall under aquaculture or fish farming.

Option 2, "Culture of silver carp," is also unrelated to silviculture. Silver carp is another type of fish and its culture would also be categorized as aquaculture.

Option 3, "Culture of oil-producing plants," is not directly related to silviculture either. Cultivation of oil-producing plants like soybean or rapeseed would fall under agriculture or crop farming.

Therefore, option 4 is the correct answer as it is directly associated with silviculture.