Why did President Nixon make efforts to improve relations with Communist China?A) to open the Chinese market to American products B) to ensure...

Of the options that are given in this question, the best
answer is D.  The main reason why Nixon wanted to improve relations with China was so as
to put pressure on the Soviet Union to also improve relations with the
US.

During the late 60s and early 70s, relations between
China and the Soviet Union were tense.  Knowing this, Nixon tried to play the two off
against one another by opening relations with China.  He reasoned that a thaw between
the US and China would worry the Soviets.  The Soviets would, Nixon thought, worry that
the US and China would act together against the Soviet Union.  It was largely for this
reason that Nixon tried (and succeded) to improve relations with
China.