Step 1: Identify the correct idiomatic usage.
The phrase “Fate smiles on someone” is a common English idiom meaning fate favors or blesses someone.
Step 2: Test the options in the sentence.
- “Smiles with” — awkward, non-idiomatic.
- “Smiles over” — incorrect in this context.
- “Smiles on” — correct idiomatic expression.
- “Smiles round” — does not fit.
Step 3: Final selection.
The only correct and natural-sounding choice is “on,” making the sentence: “Fate smiles on those who untiringly grapple with the stark realities of life.”
\[
\boxed{\text{(c) on}}
\]