You would find it much easier to compare these fractions if you'd work with a common denominator. In this problem the common denominator is 16 (actually, the LCD is 16*7, or 112, but let's continue anyway):
Then we'll be working with 8/16: a.6/16 b.10/16 c.5/7 d.9/16, instead of with 1/2 a.3/8 b.5/8 c.5/7 d.9/16.
Which fraction is less than 1/2? Ask yourself: which fraction is less than 8/16? Only (a) satisfies this: 6/16, or 3/8, is smaller than 1/2.