Question

Which missing item would complete this beta decay reaction?

Answer 1

The question is incomplete. Complete question is attached below:
.................................................................................................................

Correct Answer is : 54 131Xe

Reason: 
Emission of beta particle from radioactive material increases atomic number by one, but atomic mass number donot change.

In present case, the reactant is 53 131 I. When, iodine emits beta particle i.e. electron, it atomic number increases by one. Hence, it gets converted into Xe. Atomic number of Xenon is 54. 

Answer 2

Since you did not write down the eaction,

The Beta Decay Reaction NEVER change the total numbers of the nucleons,

for example :

131                                  131            0          0
        I      >>>>>>>>>>          Xe  +     e    +     v
   53                                    54           -1          0 

(see that the total proton and electron is still 131 and 53)