Translation:Andika kwa kifupi
Only if you were in a very specific, narrow context, like a writing teacher commenting on a student's long paper, could you translate "Write briefly" as "summarize." This example doesn't even belong in this adverb section. I suggest the correct answer is the command, "Jumlisha!"
While I agree that translating "summarize" as "write briefly" only works in certain contexts, "jumlisha" means "sum," as in "add/count up," rather than "summarize."
I believe that if translating "andika kwa kifupi" as "summarize" is acceptable because context could convey that meaning, then in the same way, "summarize" should accept a broad range of definitions that could mean "summarize" in context (e.g. "sema kwa kifupi" - "speak tersely"), assuming that Swahili does not have its own word for "summarize" (I have not looked).
Considering that this practice is in the adverb section, it may be more sensible to promote "write briefly" as the translation, and to change the English for this practice to: "Write briefly."