Step 1: Understanding the Concept
Sexual reproduction mixes genes, so offspring are not exact copies of their parents.
Step 2: Term
These differences are called variation.
Step 3: Compare the options
Heredity is the transmission of traits, mutation is a sudden change in DNA and cloning gives identical copies.
Final Answer:
The term is variation.
\[ \boxed{\text{(D) }\text{variation}} \]