The correct option is B It links to "great" and "gave" by alliteration.
Here, the use of 'glad' implies that Alcestic's husband was happy on seeing an image of her. It does not serve to imply that the husband is overwhelmed or that the husband is a particular person. The use of 'glad' also does not serve to distinguish between 'Jove's great son' and the husband (quiet evident within the context of the poem that they are two different person) and 'glad' also does not allude to the 'I' of line 1. The use of 'glad' instead of happy or joyous simply serves to link 'great' and 'grave' by alliteration (which is the occurrence of the same letter or sound at the beginning of adjacent or closely connected words and is used to create a musical effect that would enhance the reading experience)