Stephen Hawking believes that earth is unlikely the only planet ____life has developed gradually.

定语从句的先行词是the (only) planet 唯一星球

放到从句中，看起什么作用：

life (of the planet) has developed gradually 星球的生命逐渐进化

是名词的所有格，形容词性定语，因此用whose, = of which，选D