The correct answer to the question is option 3: a narrow strip of land dividing two oceans. An isthmus is a narrow land bridge that connects two larger land masses and separates two bodies of water, typically two different oceans or seas. It is important to understand the distinction between an isthmus and other geographical features mentioned in the options. Option 1 refers to the mouth of a river where it meets the sea, which is not what an isthmus is. Option 2 describes a narrow strip of water, like a strait or a channel, which is not the same as an isthmus. Option 4 is similar to the correct answer but it is not specific enough, as an isthmus specifically connects two larger areas of land, not any two random land masses.