A1 → Name     B1 → Marks     C1 → Result
A2 → Arun     B2 → 85        C2 → 
A3 → Kavi     B3 → 40        C3 → 

from openpyxl import load_workbook

wb = load_workbook("marks.xlsx")
sheet = wb.active

marks = sheet["B2"].value

if marks >= 50:
    sheet["C2"] = "PASS"
else:
    sheet["C2"] = "FAIL"

wb.save("marks.xlsx")

for r in range(2, sheet.max_row + 1):
    marks = sheet[f"B{r}"].value

    if marks >= 50:
        sheet[f"C{r}"] = "PASS"
    else:
        sheet[f"C{r}"] = "FAIL"

wb.save("marks_updated.xlsx")

Sheet1: Data

Name | Marks
Arun | 85
Kavi | 40

Sheet2 : Result

Name | Result

data_sheet = wb["Data"]
result_sheet = wb["Result"]

for r in range(2, data_sheet.max_row + 1):
    name = data_sheet[f"A{r}"].value
    marks = data_sheet[f"B{r}"].value
    if marks >= 50:
        result = "PASS"
    else:
        result = "FAIL"
    result_sheet.append([name, result])

wb.save("result_sheet.xlsx")