I think the correct answer is option C. The formula that can be used to determine the concentration of hydronium ions, [H3O+], would be [H3O+] = antilog(–pH). This is from the expression of pH which is a measure that is used for the acidity of a certain solution based on the hydronium ion concentration in the solution. It is expressed as pH = -log [H3O+]. Manipulating the equation so that [H3O+] is left on one side would lead us to the same expression as given in option C.
pH = -log [H3O+]
-pH = log [H3O+]
10^-pH = 10^log [H3O+]
10^-pH = [H3O+]
where 10^x is the same as antilog of x.
[H3O+] = antilog (-pH)
