# In which one of the following pairs,molecules/ions have similar shape?

$\begin{array}{1 1}(a)\;BF_3\text{ and t-butyl carbonium ion}\\(b)\; CO_2\;and\;H_2O\\(c)\;CCl_4\;and\;PtCl_4\\(d)\;NH_3\;and\;BF_3\end{array}$

$O=C=O\Rightarrow$ Linear
Hence (a) is the correct answer.