Answer: All the flasks contain same number of molecules.
Step-by-step explanation:
STP conditions are called as standard temperature and pressure conditions.
At this condition, 1 mole of a gas occupies a volume of 22.4 L
Also,
The temperature at this condition is taken as 273.15 K and the pressure at this condition is taken as 1 atm.
According to mole concept:
1 mole of a gas occupies
number of molecules
We are given:
Three different flasks having three different gases at STP
So, all the flasks will have same number of molecules that is

Hence, all the flasks contain same number of molecules.