Assuming ideal behavior, what is the density of argon gas at stp, in g/l?

The standard Temperature and Pressure of a gas are 273.15 K and 1 atm respectively.
Argon exists as a monoatomic atom and has a atomic mass of 39.04 g/mol.

If the gas is acting Ideally then, According to Ideal Gas Equation density is given as,

                                        d  =  P M / R T           (1)
Where,
           R  = Gas Constant =  0.08206 atm.L.mol⁻¹.K⁻¹
Now,
Putting Values in eq. 1

      d  =  (1.00 atm × 39.04 g/mol) ÷ (0.08206 atm.L.mol⁻¹.K⁻¹ × 273.15 K

                                     density  =  1.74 g/L

There is no reason to think the results are biased or not conducted properly but you should not assume the results are correct just because they were done in a laboratory so the answer would be c. it is likely to be reliable because the results support the conclusion presented.