Step 1: Understand the question.
We must recall the name Darwin gave to large, sudden variations, which are called macro-variations.
Step 2: Know what macro-variations are.
Macro-variations are big, sudden changes that appear in offspring, not the small gradual differences seen in most members of a species.
Step 3: Recall Darwin's term.
Darwin called these large sudden changes the sports of nature. He felt they were unusual jumps that nature sometimes throws up.
Step 4: Rule out discontinuous variation.
Discontinuous variation is a later genetics term, not the name Darwin himself used for these macro changes.
Step 5: Rule out the mutation choices.
Chromosomal mutations and gene mutations are modern molecular terms. Darwin lived before genes and chromosomes were understood, so he did not use these names.
Step 6: State the answer.
So Darwin called macro-variations the sports of nature.
\[ \boxed{\text{Sports of nature}} \]