AP Biology Question about phenotype probability and dominant and recessive?

Let's start by assigning letters to your traits:

Solid color = S,  striped = s
Long hair = L, short hair = l

Your cross:  long haired (could be LL or Ll) solid color (SS or Ss) x short haired (ll only) striped (ss only).  You'll get more variation in the phenotypes if you used the heterozygous parent - that is you'll get a bigger variation when crossing (using a single trait as an example) ss with Ss than with SS.  The first will give offspring that are Ss and ss while the second will only result in offspring that are Ss.  So if wanting the greatest variation in the phenotypes of the offspring, the cross would be LlSs x llss.

Now to figure out how many phenotypes are produced.  The recessive parent can only produce gametes which are ls.  The heterozygous parent can produce four types of gametes.  Here's where you use FOIL (First, Outside, Inside Last letters of each pair of letters).  For the genotype LlSs the gametes which can be produced are LS, Ls, lS, and ls.  Now set up your Punnett square to find genotypes and phenotypes of the offspring:

........|...ls...|
..LS..|..LlSs.| -> long/solid
..Ls..|..Llss..| -> long/striped
...lS..|..llSs..| -> short/solid
...ls...|..llss..| -> short/striped

So the correct answer is b. 4.