C

Step 1: Understand the process of beta decay. In beta decay, a neutron in the nucleus decays into a proton and emits a beta particle (which is an electron).
Step 2: Identify the change in the atomic number and mass number. The atomic number of the daughter nucleus increases by 1 because a neutron is converted to a proton, while the mass number remains the same.
Step 3: Therefore, for a given parent nucleus with atomic number Z and mass number A, the daughter nucleus will have atomic number Z+1 and mass number A.
Thus, the parent nucleus and daughter nucleus can be represented as Parent: (Z, A) → Daughter: (Z + 1, A).
Therefore, the parent and daughter nuclei differ by one proton.