The reaction of Lithium di(3-pentyl) cuperate with ethyl bromide results in the production of an alkane with the molecular formula C7H16. We need to determine the structural formula of the product.

Lithium di(3-pentyl) cuperate is an organocopper compound, which can be represented as LiCu(C5H11)2. Ethyl bromide, on the other hand, has the molecular formula C2H5Br.

The reaction between these two compounds involves the substitution of the bromine atom in ethyl bromide with the alkyl group from the lithium di(3-pentyl) cuperate. This type of reaction is known as a nucleophilic substitution reaction.

To determine the structural formula of the product, we need to consider the alkyl group from the lithium di(3-pentyl) cuperate. The alkyl group is 3-pentyl, which means it consists of a pentane chain with an additional methyl group attached to the third carbon atom.

Therefore, the alkyl group from the lithium di(3-pentyl) cuperate can be represented as -CH2CH2CH(CH3)CH2CH3.

Now, let's consider the nucleophilic substitution reaction between the lithium di(3-pentyl) cuperate and ethyl bromide. The bromine atom in ethyl bromide is replaced by the alkyl group from the lithium di(3-pentyl) cuperate.

The product of this reaction will have a pentane chain with an additional methyl group attached to the third carbon atom, just like the alkyl group from the lithium di(3-pentyl) cuperate. Therefore, the structural formula of the product is 3-ethylpentane (option C).

In 3-ethylpentane, the pentane chain consists of five carbon atoms connected in a straight line, with the methyl group attached to the third carbon atom. The ethyl group is attached to the third carbon atom as well.

Thus, the correct answer is option C, 3-ethylpentane.