Final answer:
The main subject of the sentence is Melinda as the sentence describes her feelings about her pet cats.
Step-by-step explanation:
In the sentence "Melinda, a new girl in my class, feels proud of her playful pet cats Fred and Ty," the main subject is Melinda. This is because the sentence is describing how Melinda feels about her pet cats. The other options provided, such as 'new girl', 'cats', and 'feelings', serve as additional information related to the main subject but they do not replace Melinda as the core focus of the sentence.