You are already given the formula and all the inputs it needs, so you only need to plug in the values.
In the formula
[tex] U = \frac{5}{2}nRT [/tex]
n is the number of mols, which you are given: 3. R is a constant, which value is given: 8.31 J(mol*K). Finally, you are given the temperature in celsius, which is 25, but you are reminded that to compute the same temperature in Kelvin you need to add 273, so you have
[tex] 25^\circ\text{C} = 25+273 = 298K [/tex]
Now we're ready to plug the values in the formula:
[tex] U = \frac{5}{2}\cdot 3\text{mol} \cdot 8.31 \frac{J}{\text{mol}\cdot K}\cdot 298K = \frac{5}{2}\cdot 3\cdot 8.31\cdot 298 J = 18572.85 J [/tex]
So, the option that best approximate the real value is the second one, 18600.
