那不然呢 is a rhetorical “what else would it be?” used when the answer feels too obvious to explain.
那不然呢？ does not supply new information. It looks at the evidence already sitting between two people and says, “What other answer could there be?”
That can be a tiny shared joke or a tiny slap. Close friends often use it when someone has been caught in an obvious act; with a sincere question, the same shortcut sounds like impatience.
Qiao
冰箱里那块蛋糕是不是你吃了？
Bīngxiāng lǐ nà kuài dàngāo shì bú shì nǐ chī le?
“Did you eat that slice of cake in the fridge?”
Zhen
那不然呢？家里还有第三个人吗？
Nà bùrán ne? Jiālǐ hái yǒu dì-sān ge rén ma?
“Who else would it be? Is there a third person living here?”
Qiao
行，明天你补一块。
Xíng, míngtiān nǐ bǔ yí kuài.
“Fine. You’re replacing it tomorrow.”
The empty plate and only two people in the apartment make the answer embarrassingly obvious. 那不然呢 turns the confession into cheek rather than a serious denial.
roommates