Final answer:
The 'Syn conformer' is not a recognized stable conformation of butane. Butane's conformations include staggered, eclipsed, anti, and gauche, with the cis conformation being a high-energy state where methyl groups eclipse each other.
Step-by-step explanation:
The question you asked is about the various conformations of butane. Butane (CH3CH2CH2CH3) has several different conformers that result from the rotation around the C-C bond in the molecule. These include the staggered, eclipsed, anti (or antiperiplanar), and the gauche conformers. However, based on the information provided, syn conformer is not traditionally recognized among the stable conformers of butane. Instead, butane features a syn conformation called cis, which occurs when the methyl groups are eclipsing each other and is the highest energy conformation. Therefore, the correct answer is '4) Syn conformer.'