Step 1: Cowpea, being a legume seed, stores carbohydrate mainly as starch/fibre, not free sugars, so its polysaccharide : monosaccharide ratio is high.
Step 2: Statement (I) is therefore true.
Step 3: Cowpea protein is known to be comparatively rich in lysine, leucine and phenylalanine, so Statement (II) is also true.
Step 4: Both statements hold, so the correct option is \( \boxed{\text{Both Statement (I) and Statement (II) are true}} \).