Which of the following applies to a solution with a pH of 4.0?
a. It is a base
b. It could be produced by the hydrolysis of sodium carbonate.
c. It has a hydroxide ion concentration of 1 x 10-14 M
d. It has a hydrogen ion concentration of 1 x 10-4M.

Hello again

The answer is
d. It has a hydrogen ion concentration of 1 x 10-4M.

"pH is defined as the negative logarithm to the base 10 of molar
concentration of hydrogen ions.

pH = - log10 [ H+ ] = log10

For pure water and neutral solution [ H+ ] = 1 ´ 10- 7

pH = log10 [H+ ] = - log (1 ´ 10- 7) = 7  "

Chemistry Study Guide
http://www.pinkmonkey.com/studyguides/subjects/chem/chap12/c1212801.asp

Note:
10^-4 may not seem like much, but since the OH- concentration of the
same solution is 10^-10 [becasue H+ * OH- = 10^-14] and
the number 10^-4 is 10^6 or 1,000,000 times larger than 10^-10
therefore
you can see the H+ concentration is 1,000,000 times greater than the
OH- concentration

Search terms used:
ph ion concentration

-Richard