Answer: A beta particle
Step-by-step explanation:
A beta particle is simply a high energy electron that is emitted from the nucleus. It DOES NOT affect the mass number of radioactive element TC, but affects the ATOMIC NUMBER by increasing it by 1 in Ru.
Thus mass number 99 remain the same in TC and Ru, but Atomic number increases from 43 to 44 due to beta decay
So, a beta particle is missing