The logarithmic function $f(x) = \log_a x$ is defined for which conditions?

• a$a > 0,\, a \neq 1,\, x > 0$
• b$a > 0,\, a \neq 1,\, x \in R$
• c$a$ any real, $x > 0$
• d$a > 1,\, x > 0$ only