To my knowledge, i is almost always used for "it" unless there is some context. In this lesson, we find "Inatosha," and there are so many other Swahili expressions that use i : haiwezekani (it's impossible), haifai (it is not proper/fitting), inasemekana (it is said that ...), inakisiwa (it is estimated that ...). I do see li being used with no context when it refers to "jambo". Proverb: "Lisemwalo lipo; kama halipo, lipo njiani linakuja" meaning roughly "The thing that is being talked about is here; if it isn't here then it is on its way it's coming." Another common phrase "Lipi kubwa la mno?" meaning roughly "What's the big deal?" But it does not seem likely that "linaingia" refers to "jambo" in this exercise.
Bottom line: "Inaingia" should certainly be accepted here, and you could argue that any inanimate singular prefix should be accepted.
The verb form generally consists of a subject-tense marker-verb (other infixes are also possible for objects and relatives, but those will come in later sections). In this case, subject = li (it, for a noun in the ji/ma noun class), tense marker = na (meaning present tense), verb = ingia (to enter).
So, the "li" in "linaingia" tells you the noun class of the subject that it is referring to in this context.