C. Net Pay is how much you've made during a pay period, YTD Net Pay is how you've made this year.
We might also note the difference between net pay and gross pay.
Gross pay is how much you earned before any taxes and deductions, net pay what you take home after taxes and deductions.
Let's use an example, and for the sake of state tax numbers, let's say you live in Minnesota. If you had a job where your total amount earned in a week (gross pay) was $1,000, and you got a weekly paycheck--assuming you are single and claiming no special tax deductions:
Federal income tax would be about $125
Social Security withholding about $60
Medicare withholding about $15
State tax withholding about $60
... So, not counting any other deductions you might include (such as your portion of health or dental insurance premiums, etc), with a gross pay of $1,000 your net pay would be about $740 per week. Year-to-date numbers would track that net pay total as it accumulated throughout the tax year.