Here are the answers for the questions provided:

**Question 15:**
The word that differs in pronunciation is **C. means**. The other words have the "s" sound pronounced as /s/, while "means" has the "s" pronounced as /z/.

**Question 16:**
The word that differs in pronunciation is **C. says**. It has a different vowel sound compared to the others.

**Question 17:**
The word that differs in stress pattern is **C. online**. It has the stress on the second syllable, while the others have stress on the first syllable.

**Question 18:**
The word that differs in stress pattern is **A. sofa**. It has a different stress compared to the others.

So, the answers are:
- Question 15: C
- Question 16: C
- Question 17: C
- Question 18: A