Final answer:
Transcriptional activators mainly modify chromatin structure using chromatin remodeling complexes and through histone acetylation to enhance gene expression.
Step-by-step explanation:
Transcriptional activators modify chromatin structure predominantly through d) Chromatin remodeling complexes. These complexes rearrange the chromatin to either expose or hide the DNA from the transcriptional machinery, thereby controlling whether a gene is active or repressed. Histone acetylation is another important method by which transcriptional activators facilitate a more open chromatin conformation, encouraging gene expression. On the other hand, histone deacetylation and histone methylation can have variable effects on gene expression, sometimes resulting in repression.