Craig Venter’s group chose to create a synthetic life-form using a bacterium as their test subject. They inserted synthetic DNA from the bacterium Mycoplasma mycoides into a Mycoplasma capricolum cell whose own DNA had been removed. The new cell accepted the synthetic DNA and was able to replicate, representing the first synthetic life-form. If they were to try the same experiment in an animal cell, what organelle would they need to replace?

The team would have to replace the nucleus.

Explanation:

Prokaryotic cells, such as the Mycoplasma capricolum cell used in the experiment do not contain either membrane bound organelles or a defined nucleus. Prokaryotic DNA floats around freely in the cytoplasm in a region called the nucleoid.

The genetic material of eukaryotic cells is protected by a membrane bound nucleus. Therefore, in order to replace an animal cell's DNA, the whole nucleus has to be removed.

Example:

In the process of cloning, the oocyte (egg cell) that receives the nucleus (from somatic cell) of the desired species or individual has to be enucleated i.e. its own nucleus has to be removed. This process is called somatic cell nuclear transfer.

that is wrong, its not thymus.