To determine the correct sequence of sentences, it's important to focus on the logical flow and continuity of ideas. Here's an analysis of the options:

- **c.** Opens by introducing the role of grandparents in an extended family, setting the stage for how they maintain unity.
- **b.** Continues by discussing how grandparents provide advice and foster tradition and cultural identity, connecting to the unity mentioned in c.
- **e.** Discusses their experience and guidance, which aligns with the advice aspect mentioned in b.
- **a.** Talks about the enduring presence of grandparents strengthening bonds, a natural follow-up to their guidance and fostering of tradition.
- **d.** Concludes with how their influence helps preserve values for future generations, a theme present in the continuity and nurturing from earlier sentences.

Considering this analysis, the option that best maintains the logical flow is **B. c – b – e – a – d**.