Names are complicated, and some people are very touchy about getting theirs exactly as they want it. In English-based naming, this can often include prefixes (e.g., "Sir" in "Sir Walter Scott") and suffixes (e.g., "Jr." as in "Martin Luther King, Jr.").
This form is making things more complicated and confusing by combining two orthogonal categories of suffix:
- Generational suffixes: I, II, III, IV, Jr., Sr., V.
- Academic suffixes: M.D., P. Eng, PE, PhD
And what should you do? Just leave this field blank.
If your name had a generational suffix, you wouldn't be asking this question.
From your comment, you don't have an academic suffix yet.
Even if you did have an academic suffix, you might not want to add it anyway. In many parts of American scientific or professional society, for example, people do not bother to use their academic titles (it's always a little weird to me to be addressed as "Dr. Beal", and I would never ask somebody to call me "Jacob Beal, Ph.D."). In many parts of Europe, on the other hand, such titles are used much more commonly and comfortably.