Which verb form correctly completes this sentence?

es improbable que muchas personas en la medicina tradicional.
a)cree
b)creen
c)crean
d)crea

Crean is the conjugation of the verb creer for the third person plural in the subjunctive mood. We use the subjunctive to express desires, doubts, the unknown, the abstract, and emotions. We know this sentence stands for the subjunctive mood because it uses the word que as a relative pronoun. Finally:

Es improbable que muchas personas crean en la medicina tradicional.

same as the person below me

there is no "following"