Answer :
B) 50% chance of the child having the disease.
This is because the parent who is a carrier must have Hh. The parent who has the disease must be hh (as it's a recessive disease, so you can only have it if you have no H).
If you draw this punnet square and fill in the blanks:
x H h
h
h
You will get:
1) Top left = Hh (child does NOT have the disease)
2) Bottom left = Hh (child does NOT have the disease)
3) Top right = hh (HAS disease)
4) Bottom right = hh (HAS disease)
Therefore 1 in 2 chance (50%)
This is because the parent who is a carrier must have Hh. The parent who has the disease must be hh (as it's a recessive disease, so you can only have it if you have no H).
If you draw this punnet square and fill in the blanks:
x H h
h
h
You will get:
1) Top left = Hh (child does NOT have the disease)
2) Bottom left = Hh (child does NOT have the disease)
3) Top right = hh (HAS disease)
4) Bottom right = hh (HAS disease)
Therefore 1 in 2 chance (50%)