How did russia’s withdrawal affect the german war effort?

the withdrawal allowed germany to make great gains on the western front.
the withdrawal inspired germany to start negotiating for peace.
the withdrawal enabled germany to focus its efforts on the ottoman empire.
the withdrawal enabled germany to focus its efforts on the western front.

I believe the answer is:  The withdrawal enabled Germany to focus it's efforts on the western front

Germany is located in the middle of allied forces in western europe and the soviet union in eastern europe.

If, both the allied forces and the soviet union attacked at the same time, the Germany would had very little chance of winning the war. Because of this, Hitler proposed a peace treaty with soviet union to not initiate a war for the next 10 years, so his army can focused on handling allied forces on western front.