Answer
Particle H created track 7.
Work Step by Step
The particle that created track 8 is known to have a seriousness quantum number of zero.
Then the particle that created track 8 is D or F.
D has a charge of -1 and F has a charge of 1.
According to the direction of curvature, only two other particles have the same charge as the particle that created track 8.
Thus the particle that created track 8 must have a charge of -1.
Particle D created track 8.
The particle that created track 9 must also have a charge of -1.
The sum of the charges of the particles that created tracks 7,8, and 9 must be positive.
Therefore, the particle that created track 7 must have a charge of at least +3.
Particle H created track 7.
