The given sentence is: "We lost confidence in him because he never __________ the grandiose promises he had made." We need to fill in the blank with the most appropriate word or phrase from the options provided.

Let's analyze each option:

1. delivered: This option is incomplete for the context. We generally use "delivered" to mean fulfilling a promise or obligation, but it often requires a preposition to indicate what was delivered, especially in this context. Thus, it doesn't fit well with the given sentence as it loses the contextual meaning without the preposition "on."
2. delivered on: This phrase means fulfilling or carrying out promises or obligations. When we say someone "delivered on promises," it means they accomplished what they promised. Therefore, "delivered on" is suitable for completing the sentence as it indicates the person failed to fulfill the promises made.
3. forgot: This option means failing to remember. While forgetting could indirectly lead to not fulfilling promises, it does not directly align with the sentence's implication of not fulfilling promises. Hence, it's less appropriate.
4. reneged on: This means going back on a promise or breaking an agreement. While it fits the context, it carries a stronger negative connotation than what is implied. The sentence focuses on a loss of confidence due to unfulfilled promises rather than intentional breaking.

Considering all options, "delivered on" is the most appropriate choice as it accurately conveys the idea of not fulfilling promises, leading to a loss of confidence. Therefore, the correct answer is "delivered on."