1. What's in your picture is NOT the Spark-Plug,
it's the Oxygen-Sensor (36533-KYJ-900).
It is recommended to replace the sealing WASHER (90454-MC7-000) and then put the O-SENSOR back in place. As with the plug, it must not be tightened beyond what is allowed.
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2. The type plug belong to Honda CBR250R (2011) is:
SPARK PLUG (SIMR8A9) (NGK)
31916-MEH-003
3. Yes, that plug is IRIDIUM Type.
4. Yes, IU24 This is the code of the equivalent NGK plug, Honda Part No 31916-MEH-003, that manufactured by DENSO
5. "A" for "A terminal nut". Remove the nut for installations on CBR250R it do not require there.