Choice B is correct. Dividing each side of the given equation by 4 yields √2x = 4. Squaring both sides of this equation yields 2x = 16. Multiplying each side of this equation by 3 yields 6x = 48. Therefore, the value of 6x is 48.
Choice A is incorrect. This is the value of 3x, not 6x. Choice C is incorrect. This is the value of 9x, not 6x. Choice D is incorrect. This is the value of 12x, not 6x.