Sonnet 18 is Shakespeare's sonnet, which means that it was written in iambic pentameter. When it comes to an iamb, we know that the sequence is as follows: the first syllable is unstressed, and the one after it is stressed. Having that in mind, the correct answers are:
2. of-
4. is
6. gold
And even though these aren't your options, there are two more stressed syllables, and those are -plex, and -dim-.