Arjuna’s last words in the Bhagavad Gita are his answer to Krishna: sthito ‘smi gata-sandehah karishye vachanam tava. “I am now free from doubts, and I shall act according to Your instructions.” It is chapter 18, verse 73.
Arjuna gives this answer at the end of a dialogue of eighteen chapters, in which he asks Krishna one question after another.