Q4. Assertion: two electric field lines can never cross each other. Reason: at a point, the net electric field has a single direction. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: at a crossing the field would need two directions at once (NCERT page 21).
Q5. Where do the field lines of a single isolated negative charge start?
At infinity: they come in radially from far away and end on the negative charge (NCERT page 21).
Q6. Why does the number of field lines crossing a sphere centred on a point charge not depend on the sphere’s radius?
The field falls as 1/r while the sphere’s area grows as r, so the number of lines crossing it, field strength times area, stays the same (NCERT page 20).