Belgian Visa D (stay fir more than 90 days)

Visa D is given ONLY to those who intend to come to settle in Belgium (work, familiy reunification, marriage, studies). In order to be able to apply and OBTAIN such visa you need to provide proofs of financial means, health insurance etc etc.
Besides visa D processing times can take several months if not years, so not worth investing in it unless there are very strong reasons.
To be noted: family reunification on ascendant line (parents joining children in Belgium to stay here) is not possible since 2011.
If your parents want to come a stay for a while (<3months)you just need to apply for visa C (tourist visa).