‘P’ will be the first part of the given sequence as ‘it is a fact’ leads to the beginning of the sentence. ‘S’ will follow next as “most prolific of food crops” makes a logical phrase. ‘R’ will come next because it says that being a prolific food crop, it yields a greater return per acre. ‘Q’ will come last because the comparative term ‘than’ shows that it is used in the context of the ‘return per acre’ in the very previous part. Hence, PSRQ is the correct sequence. Thus, option (d) is correct.