Spot the doubled-referent error and choose the standard sentence.
The mechanic who she repaired the lift called us.
The mechanic who repaired the lift called us.✓Correct answer
The mechanic which repaired the lift called us.
The mechanic who repaired it the lift called us.
Explanation
Who already stands for the mechanic as the subject of repaired, so no additional she belongs in the relative clause. Which mismatches a person antecedent, and adding it before the lift gives the verb two competing objects.