Answer: A. $A$ is the nucleon number and $Z$ is the proton number. One additional neutron increases the nucleon number by 1 while leaving the proton number, and therefore the element $X$ unchanged. The notation is ${}^{A+1}_{Z}X$

B: increases the proton number while keeping the nucleon number unchanged. C: decreases the nucleon number, corresponding to one fewer neutron at the same proton number. D: increases both the proton and nucleon numbers, adding a proton as well as a neutron.

Remember: adding a neutron changes $A$ not $Z$