Gone are __ days when we used to go fishing. ?

In the sentence “Gone are __ days when we used to go fishing,” we need a demonstrative pronoun that matches both number (plural) and distance (past, therefore “distant” in time). Let us compare the four choices by these two criteria:

1. Number (singular vs. plural)
- “This” and “that” are singular: they refer to one person, thing, moment or occasion.
- “These” and “those” are plural: they refer to more than one person, thing, moment or occasion.

2. Distance (proximal vs. distal)
- “This” and “these” point to something close to the speaker (in space or time).
- “That” and “those” point to something farther away or in the past/future.

In our context, “days when we used to go fishing” clearly refers to multiple past occasions plural and distant in time. Therefore:

• “This days” would be singular–plural mismatch and wrong number.
• “That days” is singular–plural mismatch and wrong number.
• “These days” is plural but suggests “present” (close in time), not appropriate for speaking of bygone times.
• “Those days” is plural and appropriately refers to times long past.

Thus the correct completion is “Gone are those days…,” which idiomatically expresses that such times have entirely passed.

Summary: We choose “those” because it is the plural, distal demonstrative pronoun matching “days” and conveying that the fishing days are now in the past.