If you found carbon 13 atom, you would know that

a. it has 13 protons                  c. it has 13 neutrons                     

b. it has 13 electrons               d. it has 7 neutrons

             Option-d is the correct answer.

Explanation:

             As we know that C-13 has a atomic mass of 13. Also, atomic mass is given as,

                 Atomic Mass  =  # of Protons  +  # of Neutrons   --- (1)

Since;

          Atomic Mass  =  13

And,

          # of protons  =  6 (as number of protons  =  atomic number)

Putting values in equation 1,

                               13  =  6  -  # of Neutrons

Solving for Number of Neutrons,

                               # of Neutrons  =  13  -  6

                               # of Neutrons  =  7

put the answers   and i could answer.