Which response best explains why promoters for the same sigma subunit do not have identical sequences?

The correct response is,
the sequences within the promoter region at -10 and -35 are the most important for recognition by the sigma subunit.
Promoter region is the DNA sequence located upstream of the corresponding gene. Promoters for genes are essential as RNA polymerase recognizes the promoter region and binds to it forming the holoenzyme complex which can inititate transcription of the gene located downstream.
Sigma factor is the transcription factor found in bacteria, RNA polymerase has a subunit for the sigma factor, this factor only can recognise the sequences in the promoter region and bind to it thus initiating transcription. 

Promoters usually have 2 parts of conserved regions, -10 element and -35 elements. Both these regions have conserved sequences.
The sigma factor is capable of identifying these conserved sequences at these particular locations of the promoter and can bind to these sequences.
once the sigma factor binds to these regions, RNA polymerase too binds and forms a transcription initiation complex and then transcription of the downstream gene is initiated.

Therefore promoters need not have identical sequences for the sigma factor to bind, as long as the -10 and -35 regions have conserved sequences the sigma factor can bind and transcription initiation will be followed.  

This enables the differential expression of genes. While the sigma promoter is canonically placed upstream of genes at position -10 and -35 across organisms, they differ slightly in sequence. This allows for the binding of sigma factors of RNA polymerase by different affinities across different gene promoters. Expression of genes will, therefore, be high where the sigma factors bind with high affinity and vice versa.