Using the octal system (base $6$ numeral system) to calculate $\left (243\right )\_6 + \left (15\right )\_6$, the answer should be． 
选项为:A:$\left (302\right )\_6$  B:$\left (258\right )\_6$  C:$\left (331\right )\_6$  D:$\left (262\right )\_6$  请回答其中一个选项.