The correct answer is (c) - 4 moles.
Here is how this answer is calculated:
Avogadro's number is the number of molecules in a mole of any
molecular substance, and it is equivalent to 6.0221367 x 10/23
For example, see http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/idegas.html
Scroll down to "The Mole"
For the purposes of your question, we can round Avogadro's Number down
to 6 x 10/23.
You have 2.4 x 10/24 molecules of SO3. To get the number of moles,
you need to divide this number by Avogradro's Number.
2.4 x 10/24 = 24 x 10/23
So you need to divide 24 x 10/23 by 6 x 10/23. The 10/23 cancels out,
leaving 24 divided by 6 which equals 4.
Search strategy: to get the relationship between Avogadro's Number,
moles and molecules, the search used the terms: "Avogadro's number"