Answer:
The sentence contains an error in grammar. The correct sentence would be:
"Hardly had the children (a)/ left the school (b)/ when it began to rain (c)/ No error (d)"
Step-by-step explanation:
The error in the original sentence is the use of "than" instead of "when" in part (c). "Than" is used for comparisons, while "when" is used to indicate a point in time. In this case, we are referring to a specific point in time (when the rain began), so "when" is the appropriate word to use.