Step 1: Identify what the question is testing.
This is an error-spotting question built around the correct usage of "less" versus "fewer" when a number or quantity is attached to a countable noun.
Step 2: Recall the rule.
"Less" (and its comparative forms such as "not less than") is used with singular uncountable nouns or amounts, for example "not less than fifty litres" or "not less than five kilograms". "Fewer" (and "not fewer than") is used with plural countable nouns, for example "not fewer than fifty students".
Step 3: Apply the rule to the sentence.
Here the noun being counted is "passengers", which is a plural, countable noun. Since we are stating a minimum number of individual passengers, the correct expression is "not fewer than fifty passengers", not "not less than fifty passengers".
Step 4: Check the remaining segments.
"than fifty" (b), "passengers were" (c) and "killed in the bus accident" (d) are all grammatically correct and need no change.
Step 5: Conclusion.
The error lies in segment (a), "Not less", which should read "Not fewer". This matches the answer option (a).