change to plain form

I feel kinda silly asking this. I don't know why, maybe because it ought to be simpler than I make it to be?

My exercise wants this in plain form.

-Kono tokei wa ikura desu ka.

I was thinking it should become -Kono tokei wa ikura DA ka.

Then I thought further and subsequently vexed myself whether like an I-adj I should drop the copula desu altogether to get
-Kono tokei wa ikura ka.

Then after all that thinking I thought maybe I got the whole thing wrong after all and there is another form of 'ikura' I should know and use instead because in an earlier example question the book converted 'nihongo ga wakarimasu ka' to 'nihongo ga wakaru?'

So yeah. -facepalms-

The way the exercise was written out must have confused you - the exercise is to convert the whole sentence from polite SPEECH to plain/informal SPEECH, not necessarily using plain/dictionary FORM of a verb :)

There is no informal form of "ikura" - that is simply a question word "how much," and does not change form depending on the speech style.

It should be:
- Kono tokei wa ikura?

Ah! I see the light now! -shoots fulminating glare at the book- Thank you so much! Now I have to redo the OTHER questions. -glares again-