Which of the following represents positron emission? select one: a. 3015p > 3014si + b. 23892u > 23490th + c. 147n + 42he > 178o + d. 23490th > 23491pa +

The simple trick which one can consider in such problem where it is asked for positron emission is :
When the atomic number goes DOWN by one and mass number remains unchanged, then a positron is emitted.

a.  

Here the atomic number decreases by one.

Similarly, options b and d are eliminated.

Option c is also not the answer.
For c, Count the atomic number on left side and compare it with right side. You will see it is 9 on left and 8 on right. Atomic no. did go down by 1. But the atomic mass is changed as well.

They carry out the exchange of gases as we breathe ( oxygen and carbon dioxide).