The correct answer is C, Trade combination PQR, as it would provide the most significant increase in the firm's expected netting benefit from the current level. The netting factor is calculated using the formula:

$\text{Netting Factor} = \frac{1}{n + n(n - 1)p}$

where $n$ represents the number of exposures and $p$ represents the average correlation.

For the current position with $n = 8$ and $p = 0.28$, the netting factor is:

$\text{Netting Factor} = \frac{1}{8 + 8(8 - 1) \times 0.28} = 60.83\%$

For Trade combination PQR with $n = 13$ and $p = -0.06$, there is the most significant reduction in the netting factor, indicating the most increase in netting benefit:

$\text{Netting Factor} = \frac{1}{13 + 13(13 - 1) \times (-0.06)} = 14.68\%$

Comparing this to the other trade combinations:

• Trade combination ABC with $n = 4$ and $p = 0.25$ results in a netting factor of 66.14%, indicating a deterioration in netting benefit.
• Trade combination LMN with $n = 7$ and $p = 0.15$ shows a modest improvement with a netting factor of 52.10%.
• Trade combination TUV with $n = 15$ and $p = -0.04$ has a reasonable increase in netting benefit with a netting factor of 17.13%.

However, none of these improvements are as significant as the one provided by Trade combination PQR, making it the best choice for increasing the firm's expected netting benefit.