The correct answer is (a) 1:4.
The volume of a rectangular block is given by the formula $V = lwh$, where $l$ is the length, $w$ is the width, and $h$ is the height. If the dimensions of the rectangular block are doubled, then the new length is $2l$, the new width is $2w$, and the new height is $2h$. The new volume is therefore $V = (2l)(2w)(2h) = 8V$, where $V$ is the original volume. Therefore, the ratio of the volume of the new block to the volume of the old block is $\frac{8V}{V} = 8:1$, or 1:4.
Option (b) is incorrect because the volume of the new block is 8 times the volume of the old block, not 1/8 the volume of the old block. Option (c) is incorrect because the ratio of the lengths of the new block to the old block is 2:1, not 2:3. Option (d) is incorrect because the ratio of the widths of the new block to the old block is 2:1, not 2:5.