The correct answer is: C. Is zero in all branches
In a parallel circuit, the current is divided equally among the branches. If the main line is opened, the current will be zero in all branches. This is because the current can no longer flow through the main line, and the branches are no longer connected.
Option A is incorrect because the current will not increase in the branch of the lowest resistance. The current will be zero in all branches.
Option B is incorrect because the current will not increase in each branch. The current will be zero in all branches.
Option D is incorrect because the current will not be zero in the highest resistive branch. The current will be zero in all branches.