Step 1: Read the context.
The speaker first says they expected a difficult problem, then contradicts that expectation by saying "it isn't" and describing it as "as easy as pie".
Step 2: Recall the idiom's meaning.
"As easy as pie" is a common idiom meaning something is extremely simple or effortless to do, the opposite of difficult.
Step 3: Check this against the sentence logic.
Since the sentence explicitly says the problem "isn't" difficult, the idiom must reinforce the idea of simplicity, not difficulty. This rules out "very difficult" immediately.
Step 4: Eliminate remaining options.
"Evident" means obvious or clear, "unmistakable" means impossible to doubt or mistake, and "not worthy" means undeserving; none of these convey the sense of ease implied by the idiom.
Step 5: Conclude.
The idiom means "very simple", option (e).