In triangle abc shown below, side ab is 8 and side ac is 6

This is using the Side Splitter Theorem.  Our proportion for this particular triangle is .  Instead of showing you why all the ones that don't work dont' work, I'll show you the one that DOES work and why.  It's the last choice.  If side AB measures 8, and AD is 4, the DB is also 4 because 8 - 4 = 4.  Likewise, if AC measures 6, and AE is 3, then EC is 3 because 6 - 3 = 3.  Setting this up in our proportion, we have 4/4 = 3/3, which you could cross multiply to find that 4*3 = 4*3 or we can just use the fact that 4/4 = 1 and so does 3/3.  Just remember the way to set up the SST and you'll be fine!

5 x 3 is 15 hope this helps