Which fraction is less than 1/2 a.3/8 b.5/8 c.5/7 d.9/16

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.