Question

I don,t know which one _____ （choose)

Answer 1

to choose

which one to choose是wh- to do 结构，不是从句。
1.该结构起名词作用，作主语、宾语、表语、同位语。
I didn't know how to choose .

2.wh-词如果是副词，如where\when\how等，作to do 的状语；如果是代词，如what\whose 等,作to do 的宾语或宾语的定语。
I don't know which book to choose.
I don't know what to choose.

Answer 2

chosen，省略了 to be

Answer 3

to choose（动词不定式短语）