The correct transformation of the given sentence into an assertive sentence is: It is a great pity that he has lost.

Here's the explanation:

1. Subject and Verb Agreement:

   • "It" serves as the subject, providing a neutral starting point for the sentence.
   • "is" is the linking verb, connecting the subject to the complement.

2. Expressing Regret:

   • "a great pity" maintains the sense of regret or disappointment present in the original sentence.

3. Connecting to the Situation:

   • "that he has lost" directly addresses the reason for the pity, tying the statement back to the context of the person losing.