Final answer:
The error with subject-verb agreement is the verb 'derives'. The correct verb should be 'derive' to agree with the plural subject 'calculators'.
Step-by-step explanation:
The error in the sentence regarding subject-verb agreement is with the verb 'derives'. The correct sentence should be: 'Modern calculators, which can compute complex sums almost instantaneously, derive from abacuses, hand-operated devices with beads that slide along rods.' The plural subject 'calculators' requires the plural form of the verb 'derive' to agree in number.
When checking for subject-verb agreement, it's important to identify the main subject of the sentence and ensure that the verb agrees in number with this subject, regardless of any intervening phrases or clauses. A helpful tip is to momentarily ignore the clause 'which can compute complex sums almost instantaneously' and see that 'calculators' is indeed the subject that 'derive' should agree with. So the verb should be plural to match the plural subject 'calculators'.