To solve this problem, let's denote the average speed on the first day as "x" mph.
On the first day, the hiker walks for a certain number of hours, and on the second day, the hiker walks for 2 hours longer. Therefore, let's denote the number of hours walked on the first day as "t" hours.
According to the problem, the hiker walked a total of 64 miles in 18 hours. We can create the following equation based on the given information:
$t \cdot x + (t + 2) \cdot (x + 1) = 64$
Simplifying this equation, we get:
$tx + (t + 2)(x + 1) = 64$
Expanding the equation, we get:
$tx + tx + 2x + t + 2 = 64$
Combining like terms, we get:
$2tx + 2x + t + 2 = 64$
Simplifying further, we get:
$2tx + 2x + t = 62$
Now, we need to use the fact that the total time spent walking is 18 hours. This gives us the equation:
$t + (t + 2) = 18$
Simplifying this equation, we get:
$2t + 2 = 18$
$2t = 16$
$t = 8$
Now that we have the value of "t", we can substitute it back into the equation:
$2tx + 2x + t = 62$
$2(8)x + 2x + 8 = 62$
$16x + 2x + 8 = 62$
$18x + 8 = 62$
$18x = 54$
$x = 3$
Therefore, the hiker's average speed on the first day was 3 mph.
Therefore, the correct answer is option C) 3.