"Your mother has a smooth forehead!"
Translation:Hab SoSlI' Quch!
We have a verb and two nouns in this sentence. Remember that "be" verbs like Hab ("be smooth") act as verbs before the noun(s) and as adjectives when following nouns. So it's the verb here. Something is smooth. Let's look at the nouns to find out what is smooth.
Two nouns next to each other usually mean the first one is describing the type or the ownership of the second noun. The last noun in such a string of nouns is the actual thing we're talking about. So we are talking about a Quch here - a SoSlI' kind of Quch - "a your-mother forehead" or "your mother's forehead".
That all results in, "Your mother's forehead is smooth." Which is also listed as an alternant best translation in the reverse of this exercise. However, in other Klingon materials the translation of this phrase is usually given as "Your mother has a smooth forehead," so we list that as the first best translation and thus that is the English phrase you are presented with in this exercise.
We do also allow the more literal Quch Hab ghaj SoSlI', and a couple other variations as accepted answers on this exercise.
It is especially confusing because you are being asked to translate a sentence in English that contains the verb "to have" (has) and that verb is not available as an option. What they should present in English (based on the "correct" answer) is "Your Mother's forehead is smooth." These are two different ideas! This is more easily illustrated if we switch out the noun "forehead" with "painstick" (to keep it Klingon). Now notice the difference: "Your Mother's painstick is smooth" i.e. your mother owns a smooth painstick; whereas "Your Mother has a smooth painstick" could mean that the painstick currently in her hand is smooth - even though that painstick might belong to someone else! To bring this back around, perhaps that smooth forehead your mother "has" is on the head of a decapitated Terran (while her own forehead is just as ridgy as the next Klingon's).