Final answer:
Karl has a net short-term capital gain of $12,000 after offsetting his capital gains and losses against each other.
Step-by-step explanation:
To calculate Karl's capital gains and losses, we need to net his long-term capital gains (LTCG) and losses (LTCL) together as well as his short-term capital gains (STCG) and losses (STCL). Karl's LTCG is $6,000, and his LTCL is $12,000. When these are netted together, Karl has a net LTCL of $6,000 ($6,000 LTCG - $12,000 LTCL). Similarly, Karl's STCG is $30,000, and his STCL is $18,000. When netted, Karl has a net STCG of $12,000 ($30,000 STCG - $18,000 STCL).
Next, we need to net the short-term gains against the long-term losses. Karl has a net STCG of $12,000 and a net LTCL of $6,000. His net capital gain is therefore $6,000, which is the difference between the net STCG of $12,000 and the net LTCL of $6,000.
Thus, the correct answer is C. $12,000 net short-term capital gain.