The fastest solution involves simply checking each answer choice.

Here's the (slower) algebraic solution:

Let x = the tens digit of the number
Let y = the units digit of the number

So, the VALUE of the number in question = 10x + y (in the same way that the VALUE of 27 = (2)(10) + 7)

The value of a two-digit integer is twice as many as the sum of its tens digit and units digits.
We can write: 10x + y = 2(x + y)
Simplify right side: 10x + y = 2x + 2y
Subtract y from both sides: 10x = 2x + y
Subtract 2x from both sides: 8x = y

This tells us that the units digit (y) is 8 times the value of tens digit (x)
Since 18 is the only number that satisfies this condition, the correct answer must be C