Select the option that displays the grammatically correct plural form (Bahu-vachan) of the word 'दवाई' (Dawai - Medicine):

According to Hindi grammar rules for changing number ($\text{Vachan}$ modification):
When a feminine noun ends with a long '$\text{I}$ ' sound ($\text{ई}$), like दवाई, while forming its plural:

1. The long vowel '$\text{ई}$ ' shifts to a short vowel '$\text{इ}$ '.
2. The suffix '$\text{याँ}$ ' with a chandrabindu is appended to the end.

Thus, $\text{दवाई}\to \text{दवाइयाँ}$. Option B represents the absolute correct spelling.