An idiomatic expression is a type of informal English where the meaning
of the expression is different than the literal meaning of the
expression.
Option A: 'like the back of my hand' is an idiom that means 'to have very detailed knowledge about something' (the same way you know the back of your hand very well). It relates to the sentence, which means: the science test was very easy because the person knew the answers having detailed knowledge about them. Hence A is correct.
Option B: The idiom 'like it was yesterday' is used to say that something seems like it just happened. For example, 'The events are so fresh in my mind, like it happened yesterday.' It does not relate to the context of the conversation, hence B is incorrect.
Option C: 'There's no tomorrow' means 'doing something without proper thought, hurriedly'. It does not relate to the text, which concentrates on how easy the test was. Hence C is incorrect.
Option D: 'It's now or never' means 'something must be done immediately, or else the chance would be lost'. For example, 'You must participate in the contest. It's now or never.' It does not relate to the sentence, hence D is also incorrect.