Answer
See below
Work Step by Step
Let $A$ and $B$ be $n \times n$ matrices.
If $A$ is skew-symmetric, then $A^T=-A$
Obtain: $(B^TAB)^T\\=B^TA^T(B^T)^T\\=B^TA^TB\\=B^T(-A)B\\=-(B^TAB)$
Hence, $B^TAB$ is skew-symmetric.
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