Is tetraammine dichloro cobalt (III) an optically active compound?
$\ce{Co[(NH3)_4Cl2]}$ can exist as cis and trans isomers as shown in the following figure.



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If a molecule contains a plane of symmetry it cannot be optically active or chiral.

In the cis isomer there is a plane of symmetry containing the $\ce{Cl-Co-Cl}$ atoms, therefore this isomer cannot be optically active.  With the trans isomer there are several planes of symmetry (one contains the $\ce{Co}$ atom and the four $\ce{NH3}$ groups, and there are four more planes of symmetry containing the $\ce{Cl-Co-Cl}$ atoms and either containing or bisecting the pairs of amino groups), therefore the trans isomer is also optically inactive.