Chọn phương án C.

Ta có \(u_1\cdot u_{15}=u_1\cdot u_1\cdot q^{14}=u_1^2\cdot q^{14}\).

Trong khi đó:

• \(u_2\cdot u_{14}=u_1\cdot q\cdot u_1\cdot q^{13}=u_1^2\cdot q^{14}\)
• \(u_5\cdot u_{11}=u_1\cdot q^4\cdot u_1\cdot q^{10}=u_1^2\cdot q^{14}\)
• \(u_6\cdot u_9=u_1\cdot q^5\cdot u_1\cdot q^8=u_1^2\cdot q^{13}\)
• \(u_{12}\cdot u_4=u_1\cdot q^{11}\cdot u_1\cdot q^3=u_1^2\cdot q^{14}\)

Vậy đẳng thức \(u_1\cdot u_{15}=u_6\cdot u_9\) sai.