Answer:
More.
Step-by-step explanation:
The comparative degree of adjective is primarily employed to establish the comparison between two people, ideas, or objects. It is indicated by the use of 'than' which reflects that 'one is better than the other' in quality, size, or other aspects.
In the given sentence, the correct word to reflect the comparison between two things(Mariam and Laila's sin) would be 'more' as the adjective 'forgivable' consists of more than three syllables which implies that the comparative degree would be formed using 'more'(instead of suffix '-er'). Thus, 'more' is the correct answer.