I basically agree with Robert--Us, but to make that clearer I'd like to distinguish three possible cases and their proper translation into German.
In a conditional sentence type 1 the if-clause is in the present tense, the main clause is in the future tense. The condition (if...) is regarded as quite probable (not as true; we're talking about the future) and the consequence as inevitable as soon as the condition is fulfilled. This is the case in your example.
If it rains, we will have to turn back.
a) Wenn es regnet, müssen wir umkehren.
b) Wenn es regnet, werden wir umkehren müssen.
As in English, the present indicative refers to the future (the condition is not yet true). The main clause can be in present tense (which you can use for future events in German, see "Morgen besuchen wir meine Eltern") or in future tense. Both are equally acceptable, b) is more likely in more formal contexts.
Note: "zurrückgehen" is a misspelling, it would have to be "zurückgehen", but anyway this is not idiomatic. If you want to say that we are on our way but in case of rain (say: tomorrow) will have to turn back (tomorrow) you'd use "umkehren".
There is a second case which is sometimes called a "zero conditional" in English:
If you heat ice it turns to water.
Wenn man Eis erhitzt, wird es zu Wasser.
(Note that the German "wird" is not an indication of the future tense here but the verb "becomes" in present tense indicative.)
Both in English and in German a double present tense indicative refers to an undisputed consequence of some action. Condition A leads to consequence B. This type of conditional sentence expresses a universal truth or a habitual reaction which was, has been, and will be the case (true). This one exception to Robert's "most often uses the future" is rare.
A third case (conditional type 2) is a past tense in the if-clause followed by a conditional tense (with would) in the main clause. This, in English, does not refer to the past (as suggested by the past tense) but rather to an improbable condition. This sentence, like a conditional sentence type 1, refers to the present or future. It simply says that the speaker regards the condition, and with it the consequence, as improbable:
If it rained we would have to turn back.
Same thing in German. In German you need Konjunktiv II in the if-clause. The Konjunktiv-II-forms of verbs are frequently identical with the simple past, therefore many speakers prefer the paraphrase with "würde":
Wenn es regnete/wenn es regnen würde, müssten wir umkehren/würden wir umkehren müssen.
The main clause, as you see, demands a Konjunktiv II (still sometimes called a "past subjunctive" or "Konjunktiv Imperfekt", but it has nothing to do with the past).
Your translation C is a translation of this variant and therefore not a translation of your original sentence. It's a bit unusual, too, as you use the "würde"-paraphrase in the first part but not in the second. In terms of register of speech, speakers would probably opt for the same construction in both parts (either regular subjunctives or subjunctives paraphrased with "würde").
Your version C is the translation of a different sentence and even then a bit shaky.
Your variant A is not grammatical ("If it rained, we must turn back"???).
Your variant B is correct in terms of tenses but, as I said, the "zurückgehen"-part is not idiomatic.
I hope I haven't discouraged you. You're on the right track, keep going.
P.S.: Gosh, that took me too long. I missed the last three replies. I hope my contribution is not beside the point so I'll send it unaltered.