"A dog is a pet."
Translation:כלב הוא חיית מחמד.
Read the tips and notes of Construct class:
"If the nismach is a feminine noun that ends with a "ה", the "ה" is replaced with a "ת":
A cake - עוגה (uga) Chocolate - שוקולד (shokolad) A chocolate cake - עוגת שוקולד (ugat shokolad - literally "cake of chocolate")"
Surely כלב is just as valid as כלבה as the question just says dog, and doesn't specify female dog? But it's marked as a typo if I don't use the female form.