Step 1: Read the sentence as a whole.
\textit{“Go North-East across the mountains till you will reached an island.”}
Step 2: Identify the grammatical rule involved.
After time conjunctions like till, until, when, the simple present tense is used to refer to future time.
Step 3: Locate the error.
The phrase “will reached” is incorrect because:
Future auxiliary will should not be used after till Past form reached is incorrect with will
Correct form:
\[
\text{till you reach an island}
\]
Step 4: Final conclusion.
The error lies in part \(\boxed{\text{(C)}}\).