In the following question, a sentence contains an underlined part: "Go north east across the mountains till you will reach an island." Choose the option that best improves the underlined part to make the sentence grammatically correct.

Introduction / Context:
This sentence improvement question focuses on the use of tenses and conjunctions in instructions. The original sentence gives a direction about travelling across mountains and reaching an island. The issue is with the use of will after till. Understanding how future time is expressed in clauses introduced by till, until, when, and similar conjunctions is essential to choosing the correct option.

Given Data / Assumptions:

• Original sentence: Go north east across the mountains till you will reach an island.
• The underlined part is till you will reach an island.
• Available improvements include different verb forms after till, as well as a No improvement option.
• The sentence expresses a future result of a present instruction.

Concept / Approach:
In English, when we use time conjunctions such as when, after, before, till, or until to refer to future time, we normally use the simple present tense in the time clause, not will. The future sense is understood from the main clause or overall context. Therefore, after till we should write you reach rather than you will reach. This rule applies especially in instructions and directions, where the main verb is in the imperative form such as go or turn. The grammatically correct structure is therefore till you reach an island.

Step-by-Step Solution:
Step 1: Identify the main clause and the time clause. Main clause: Go north east across the mountains. Time or limiting clause: till you will reach an island. Step 2: Recall the rule that clauses introduced by till or until referring to the future use the simple present tense rather than will plus base verb. Step 3: Replace will reach with reach to form till you reach an island, which expresses a future point in time in a grammatically correct way. Step 4: Check option B, which gives till you reach an island. This matches the required pattern and keeps the meaning unchanged. Step 5: Evaluate option A, till you reached an island, which uses simple past and does not fit with the imperative Go. Step 6: Evaluate option C, till you have reached an island, which uses the present perfect and is not natural in directions of this type. Step 7: Reject No improvement because the original expression till you will reach an island violates the standard rule for time clauses.

Verification / Alternative check:
Compare with other correct examples: Wait here till I come back, Stay here until it gets dark, Do not move until I say so. In each case, the clause after till or until uses the simple present tense even though it refers to a future action. The sentence Go north east across the mountains till you reach an island follows the same pattern. Any version with will after till would sound unusual and be marked as incorrect in standard written English.

Why Other Options Are Wrong:
Option A, till you reached an island, implies a past event and does not suit an instruction that is being given now. Option C, till you have reached an island, uses the present perfect in a way that suggests a prior condition rather than a simple future point; it is rarely used in straightforward directional sentences. Option D fails to correct the misuse of will in the original sentence, so it cannot be accepted.

Common Pitfalls:
Students often carry over the use of will from main clauses into time clauses because they know the overall event is in the future. However, English has a special rule that future events introduced by time conjunctions take the simple present in those clauses. Remembering examples such as when you arrive, call me and till you finish, stay here can help you develop a natural feel for this pattern and avoid adding will in the wrong place.

Final Answer:
The correct improvement is till you reach an island.