What affect does the "a" value have on the function f(x) = log(x)?
a. the "a" value reflects the graph across the y-axis.
b. the "a" value moves the function up or down.
c. the "a" value moves the function to the left or right.
d. the "a" value stretches the function vertically by a factor of absolute value of a.

The correct answer to this question is letter "A. The "a" value reflects the graph across the y-axis."

I would guess they're referring to some general form which is
f(x) = a log(x − h) + k

Where h represents horizontal shifts, k vertical shifts and a stretches the function.

a will also reflect the function across the x-axis if the value is negative. But NOT across the y-axis.