Concept:
Rainfall is the amount of precipitation received over a particular area during a given period of time. It is usually measured in millimetres.
Step 1: Identify the required parameter.
The question asks about the instrument used to measure rainfall.
\[
\text{Parameter}=\text{Rainfall}
\]
Step 2: Identify the correct instrument.
Rainfall is measured by using a rain gauge.
\[
\text{Rainfall} \rightarrow \text{Rain gauge}
\]
Step 3: Eliminate other options.
Moisture meter is used for measuring moisture.
Neutron probe is used for measuring soil moisture.
SPAD meter is used for measuring leaf chlorophyll content.
\[
\therefore \text{Correct Answer is (A)}
\]