Which of the following solutions is alkaline?

A solution is considered alkaline if its pH is greater than 7. pH is defined as the negative logarithm (base 10) of the hydrogen ion concentration: pH = -log10([H+]). For [H+] = 1×10^-4 Mol/l, pH ≈ 4, which is less than 7, so this solution is acidic. For [H+] < 1×10^-7 Mol/l, a lower hydrogen ion concentration indicates a higher pH; since the comparison is less than a very small value (1×10^-7), we can infer the pH is greater than 7, so this solution is alkaline.