Explanation:

Total number of possible arrangements for 4 boys and 3 girls in a queue = 7!
When they occupy alternate position the arrangement would be like:
B G B G B G B

Thus, total number of possible arrangements for boys = (4×3×2)
Total number of possible arrangements for girls =(3×2)

Required probability = 4 x 3 x 2 x 3 x 2 / 7! = 1/35

Hence, option 2 is the correct answer.