The scores of five students in a subject are entered in row B, from cells B2 to B6. The grades for the scores are as follows:

- A: 90 to 100
- B: 80 to 89
- C: 70 to 79
- D: 60 to 69
- F: 0 to 59

Which of the following functions can you insert in the D2 cell so that it returns the grade for the score in cell B2?

1. `=IF(B2<60, "F", IF(B2<70, "D", IF(B2<80, "C", IF(B2<90, "B", "A"))))`

2. `=IF(B2<=59, "F", IF(B2<=69, "D", IF(B2<=79, "C", IF(B2<=89, "B", IF(B2<=100, "A"))))`

3. `=IF(B2=0-59, "F", IF(B2=60-69, "D", IF(B2=70-79, "C", IF(B2=80-89, "B", IF(B2=90-100, "A"))))`

To determine the correct function to insert in cell D2 for returning the grade based on the score in cell B2, we need to analyze each option to see which one correctly implements the grading logic using nested IF functions in Excel.

Let's break down each of the three options provided:

=IF(B2<60, \"F\", IF(B2<70, \"D\", IF(B2<80, \"C\", IF(B2<90, \"B\", \"A\"))))
- This option correctly implements the grading scale by checking each range in ascending order. If the score in B2 is less than 60, it returns "F". If it's less than 70, it returns "D". If less than 80, it returns "C". If less than 90, it returns "B". Otherwise, it returns "A".
=IF(B2<=59, \"F\", IF(B2<=69, \"D\", IF(B2<=79, \"C\", IF(B2<=89, \"B\", IF(B2<=100, \"A\")))))
- This option also correctly implements the grading logic. It uses <= to specify the upper bound for each grade range, ending with "A" for scores 90-100.
=IF(B2=0-59, \"F\", IF(B2=60-69, \"D\", IF(B2=70-79, \"C\", IF(B2=80-89, \"B\", IF(B2=90-100, \"A\")))))
- This option is incorrect because the use of = with a range (e.g., B2=0-59) is not valid in Excel as it does not evaluate ranges correctly.

Given the analysis, options 1 and 2 are both correct and will return the appropriate grade for a given score in cell B2. However, option 1 uses fewer checks (< instead of <=) and is a common convention in Excel.

Thus, the correct choice based on the standard convention in Excel is option 1.