Concept:
Mitosis is an equational cell division in which one parent cell gives rise to two genetically identical daughter cells.
Step 1: Understand the role of mitosis.
Mitosis helps in growth, replacement of damaged cells and repair of tissues.
\[
\text{Mitosis} \rightarrow \text{growth and repair}
\]
Step 2: Check the options.
Cell reduction is related to meiosis, not mitosis.
Cell differentiation is specialization of cells.
Cell elongation is increase in cell length and is not the direct contribution of mitosis.
Step 3: Select the best option.
Among the given options, the most suitable contribution of mitosis is cell repair.
\[
\therefore \text{Correct Answer is (A)}
\]