Determine the concentration of a solution prepared by diluting 25.0 ml of a stock 0.188 m sr(no3)2 solution to 150.0 ml

We can use the following equation to calculate the concentration of diluted solution.
c1v1 = c2v2 
where c1 is concentration and v1 is volume of stock solution, which is the more concentrated solution.
c2 is concentration and v2 is the volume of diluted solution.
substituting the values in the equation,
0.188 M x 25.0 mL = c2 x 150.0 mL 
c2 = 0.0313 M
concentration of diluted solution is 0.0313 M

What is the gold assay

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.