In maize, hybrid vigor is exploited by

The correct option is D Crossing of two inbreed parental lines
Heterosis is also known as hybrid vigor or outbreeding enhancement. It is the improved or increased function of any biological quality in a hybrid offspring. An offspring exhibits heterosis if its traits are enhanced as a result of mixing the genetic contributions of its parents. These effects can be due to Mendelian or non-Mendelian inheritance. It is also the opposite of inbreeding depression although differences in these two concepts can be seen in evolutionary considerations such as the role of genetic variation or the effects of genetic drift in small populations on these concepts. Inbreeding depression occurs when related parents have children with traits that negatively influence their fitness largely due to homozygosity. In such instances, outcrossing should result in heterosis.

In maize plant heterosis exhibits. Modern maize hybrids substantially out yield conventional cultivars and respond better to fertilizer. If the two inbreed parental lines are crossed then the hybrid vigor will be lost.

Thus, the correct answer is option D.