The stopping line reaches zero at the threshold

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Problem: The stopping line reaches zero at the threshold
A metal’s stopping potential rises by 0.80 V when incident frequency increases by 2.0×10¹⁴ Hz. At frequency 8.0×10¹⁴ Hz, its stopping potential is 1.20 V.
Question: What threshold frequency follows from the measured straight-line model?
Givens:
- ΔVs = 0.80 V for Δf = 2.0×10¹⁴ Hz
- Vs = 1.20 V at f = 8.0×10¹⁴ Hz
Assumptions: Ideal photoelectric stopping relation Vs = (h/e)(f − f0); use the measured slope for this dataset, not an independently rounded physical constant. Frequencies supplied are above threshold.
Choices (LaTeX):
A. 5.0\times10^{14}\ \mathrm{Hz}
B. 3.0\times10^{14}\ \mathrm{Hz}
C. 8.0\times10^{14}\ \mathrm{Hz}
D. 1.1\times10^{15}\ \mathrm{Hz}

Ask for my approach first. Help me distinguish a physical misconception from an algebra error without revealing which choice is correct.

Tutoring approach:
Act as a patient physics tutor. Ask what I understand and what I have tried, then wait for my reply. Teach one step at a time; do not give the final answer or a complete solution immediately. Start with a physical picture. Define unfamiliar symbols and operators, explain signs and units, and state the model's assumptions. Give progressively stronger hints only when I need them. Ask me to predict a change before calculating it. When I finish, help me check dimensions and a limiting case, then ask one related transfer question. If information is missing or ambiguous, ask rather than inventing it. Distinguish exact results from approximations and acknowledge uncertainty.