The correct answer is D. It is written in iambic pentameter.
Explanation
A sonnet is a type of fourteen-line poem written in iambic pentameter. The iambic pentameter as its name implies is the sequence of five iambs. An iamb is an unstressed (symbolized with x) syllable followed by an accented one (symbolized with /).
Another characteristic of the sonnet is a structure of two quartets and two triples, that is, the first four lines rhyme, the next four lines rhyme, the next three lines rhyme, and the last three lines rhyme.
According to the above, the Sonnet XII by William Shakespeare is a sonnet because its first four verses rhyme with its last words: time, night, prime, and white. The second quartet also rhymes with words: leaves, herd, sheaves, and beard.
First triple rhyme with words make, go, forsake; second triple rhyme with words: grow, defense, and hence.
Also, if we examine any verse we can confirm that it was written in iambic pentameter, for example:
When(x) I(/) do(x) count(/) the(x) clock(/) that(x) tells(/) the(x) time(/).
So, the correct answer is D. It is written in iambic pentameter.