Among the given species  the isostructural pair is : 

1. NF3 and BF3

2. BF4- and NH4+

3. BCl3 and BrCl3

4. NH3 and NO3-

HINT: Isostructural species are those which have the same shape and hybridization.
 
Explanation:
 
1.) NF3 is pyramidal whereas BF3 is trigonal planar. Hence, both have different shapes.

2.) BF4- and NH4+ ions both are tetrahedral. Both have the same shape and the hybridization is also the same, that is, 
sp3.

3.) BCl3 is trigonal planar whereas BrCl3 is trigonal bipyramidal. Both have different same shapes.

4.) NH3 is pyramidal whereas NO3- is trigonal planar. Both have different shape.
Hence, option second is the correct answer.