Final answer:
A lawsuit filed on April 1, 2015 would trigger the Ace Insurance policy to pay.
Step-by-step explanation:
The lawsuit that would trigger the Ace Insurance policy to pay is option c) A lawsuit filed on April 1, 2015. This is because the Ace Insurance policy was in effect from March 1, 2010, until March 31, 2013. The transition to the DEF Insurance policy with a Claims Made policy occurred on April 1, 2013. Therefore, any lawsuit filed after this transition date would fall under the DEF Insurance policy.