"It is a gallant child; one that
indeed physics the subject,
makes old hearts fresh: they
that went on crutches ere he
was born desire yet their life
to see him a man."
A. The speaker believes Mamillius can
reverse age.
B. The elderly in the kingdom are holding
on to life long enough to see Prince
Mamillius grown.
C. A child caused a bunch of trouble and
many people are now on crutches.
D. The people made a child some crutches
when he was born.

The speaker is saying that the elderly in the kingdom are so impressed by Prince Mamillius that they are holding on to life long enough to see him grow up. This suggests that the elderly are finding renewed hope and vitality in the young prince.

Supporting details:

The speaker uses the word "gallant" to describe Mamillius, which suggests that he is a brave and noble young man.
The speaker says that Mamillius "makes old hearts fresh," which means that he is inspiring the elderly to feel young again.
The speaker says that the elderly "desire yet their life to see him a man," which means that they are hoping to live long enough to see Mamillius become an adult.

Option A is incorrect because there is no evidence in the passage to suggest that Mamillius can reverse age.

Option C is incorrect because there is no evidence in the passage to suggest that a child caused a bunch of trouble.

Option D is incorrect because there is no evidence in the passage to suggest that the people made a child some crutches when he was born.

Conclusion:

Therefore, the correct answer is B. The elderly in the kingdom are holding on to life long enough to see Prince Mamillius grown.

I hope this helps!