To solve this question, we need to fill in the blank with the most appropriate modal verb based on the context provided in the sentence: "They __________ be very far away. I saw them a little while ago."

1. Understand the meaning of each option:
   • Shall not: Used to express a future action or command, generally not suitable for indicating impossibility.
   • Cannot: Indicates impossibility or inability. Appropriate for expressing certainty that something is not the case.
   • Should not: Indicates advice against doing something or expressing a probability less than certainty, but greater than mere possibility.
   • Ought not: Similar to 'should not,' used to express duty, advisability, or expectation.
2. Contextual Analysis: The sentence is suggesting that "They cannot be very far away" with the reasoning that "I saw them a little while ago." This implies a strong degree of certainty that because they were seen recently, it is impossible for them to be far away now.
3. Reasoning:
   • The use of "cannot" fits perfectly as it suggests the impossibility of them being far away under the current context of recent sighting.
   • "Shall not" is incorrect because this modal is used for future intent or strong determination, neither of which relates to assessing the current situation.
   • "Should not" and "ought not" would suggest advisability or a recommendation, not impossibility, thus making them less appropriate for the context given in the problem.

Conclusion: The most appropriate option to fill in the blank is "cannot," as it correctly conveys the impossibility that the subjects are far away after recently being observed.