Translation:The first month of the year is January.
Right! I'm confused too. "des Jahres" is genitive, right? So, it must show possession when translated to English. right? "Of' was not a choice in the word bank, so I chose "in" and felt stupid.
The "Tips and notes" at the beginning of each lesson group seem to be written by people very knowledgeable in both languages. But, the sentences in the lessons and their translations don't give me the same confidence.
I would love to read an argument in support of the use of the word "in". Don't argue that it is OK to say it. Argue that it translates to "des Jahres" without some German preposition in front of it.
As a general rule, we use use [possessor]'s [possession] when the possessor is a person and [possession] of [possessor] when the possessor is inanimate, especially when it's abstract. Both ways are correct, but it's weird to say "the husband of my sister" and weirder to say "the year's first month".