The Arabic options (and "correct" definite article selection: "al-maTar al-thaqiil") does not match English "Heavy rain" (indefinite)
''Heavy rain'' is not really indefinite, it just has no article (''a/an'' is indefinite). English sentences without "a" nor "the" can be both definite or indefinite, depending on the meaning and context.
I asked a similar question in the reverse translation here
Q: Why is this not the heavy rain is amazing?
A: it can also be that. But in terms of meaning, the rain here is a 'definite type' of rain.