Solution

In DES, the round function follows these steps:

First, expansion permutation is applied to expand the right half from $32$ bits to $48$ bits.

Then XOR operation is performed with the round key.

After that, S-box substitution is applied to reduce $48$ bits to $32$ bits.

Finally, straight permutation is applied.

So, the correct order is:

$Expansion\ Permutation \rightarrow XOR \rightarrow S\text{-}boxes \rightarrow Straight\ Permutation$