Answer:
"I strongly disagree with the artist's decision to use black-and-white photography to document the fire"
This sentence is subjective in as much as the author states his/her personal opinion about the way the photographer is working. We can see this by the use of the personal pronoun "I". Moreover, his own opinion is formally expressed by the use of these words "strongly disagree".
Besides, this expression "strongly disagree" is much more formal than "I think that". In formal writing, contractions are not usually allowed, but the possessive "s" can be an exception.
The last two sentences are more formal but they are not subjective because there is no reference to whom is giving his/her opinion. In other words, the sentence presents an objective and impersonal fact.