Mensuration is the chapter

Solution:
Total Surface Area of a cube = 6s²
 $$\begin{gathered} \Rightarrow 6{\mathrm{s}}^2=100 \\ \Rightarrow \mathrm{s}=\frac{10}{\sqrt{6}} \end{gathered}$$
Now, when two cubes are join then it will be a cuboid of dimensions,
$\mathrm{length}=\frac{20}{\sqrt{6}}, \mathrm{breadth}=\frac{10}{\sqrt{6}},\mathrm{height}=\frac{10}{\sqrt{6}}$
$\begin{array}{rcl}Total Surface Area& =& 2\left(\frac{20}{\sqrt{6}}\times \frac{10}{\sqrt{6}}+\frac{10}{\sqrt{6}}\times \frac{10}{\sqrt{6}}+\frac{10}{\sqrt{6}}\times \frac{20}{\sqrt{6}}\right)\\ & =& 2\left(\frac{200}{6}+\frac{100}{6}\frac{200}{6}\right)\\ & =& \frac{2\times 500}{6}\\ & =& \frac{500}{3}\ne 200\end{array}$
Thus, the assertion is false but the reason is true.

Hence, the correct answer is option (d).
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