The road was muddy, as it had rained the day before.
Instinctively, I would prefer option 1. But it seems the canonical post cited by user 170849, and written by Stoney.B suggests that either option is acceptable.
4. How and when should I use the perfect?
... For instance, many learners are under the impression that because a past perfect is often used to speak of one event being prior to another, it should be used whenever you do so. Now it is true that there are circumstances when you must employ the past perfect:
- OK At the time his first play was produced, Shaw had already established a substantial literary reputation.
But what ‘requires’ the past perfect there and forbids a simple past is not the time sequence but the adverbials at the time his first play was produced and already, [emphasis mine] which both locate the later endpoint of the Event Time timeframe at Reference Time. Without those adverbials it is quite possible to express the same time sequence in a sentence which permits either a simple past or a past perfect:
- OKShaw had established a substantial literary reputation before his first play was produced.
- OK Shaw established a substantial literary reputation before his first play was produced.
In terms of literal meaning, these two sentences amount to the same thing.
So the content of the sentence is not a reliable guide to whether or not it wants to be expressed as a perfect construction or something else. [...]
This is why I am so fond of an answer here on ELL which I have christened FumbleFingers‘ Perfect Truism. (FumbleFingers speaks specifically of the past perfect, but the principle may be generalized).
☛ “Don’t use the perfect unless you need it.”
What governs the use of the perfect is not the content, the meaning it expresses, but the purpose it serves [emphasis mine]. If you want to know whether to use a perfect, look at what you are trying to accomplish. What really distinguishes the perfect from the deictic constructions is focus: are you talking about state of affairs current at Reference Time or are you talking about the prior eventuality which in some sense gave rise to the current state of affairs? [emphasis mine]
N.B. Where the original is not in bold, I have stated as [emphasis mine]
In the OP's example; the event (rain) specifically occurred prior to an occurrence (a muddy road). The road was muddy due to a rainfall that happened and ended at a very specific point in time: the day before. Some might name the phrase in bold an adverbial phrase or an adjunct, regardless, this phrase/adjunct unequivocally establishes the sequence of events, and in a narrative setting, the Past Perfect is preferable in this instance (especially under English exam conditions!).