Please read the instruction(s) in exercise 1.
Please read an instruction in exercise 1.
closed as too localized by FumbleFingers, RegDwigнt♦ Jan 16 '12 at 11:25
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I think the convention is to use:
So I recommend this.
In the second sentence:
"an instruction" implies that you are allowing the reader to simply choose any instruction in exercise 1 and start reading it. In most cases, this is not so. Usually when you are telling someone to read an instruction, you are talking about a specific instruction and not requesting him / her to just pick any random guideline or instruction and start reading it.