A ParaPro is choosing a prompt after seeing this student work. Problem: 769 + 187. Student work: Step 1: 9 + 7 = 16, so write 6 ones and regroup 1 ten. Step 2: 6 + 8 + 1 = 15, so write 5 tens and regroup 1 hundred. Step 3: 7 hundreds + 1 hundred = 8 hundreds, so the answer is 856. Which option identifies the first regrouping error and gives the best follow-up prompt?