Of the options given in this question, the best answer is A.

The Yalta Conference was held in February of 1945.  President Franklin D. Roosevelt was still alive at this time (he died in April of that year) and attended the conference.  This makes both C and D incorrect.  Stalin did not leave the conference and came to an agreement on the issue of Poland that was acceptable to him.  The agreement on Poland was part of the plans for post-war Europe, which basically split Europe between the Soviets and the Western Allies.  This makes B incorrect and A correct (A is also correct because Stalin agree to enter the war against Japan 3 months after Germany surrendered).