The stressed syllable is written in capital letters. radioactive

Analyze the question: You need to find which syllable of "radioactive" carries the main (primary) stress.

Concept: In English, words ending in -ive usually keep the stress away from the suffix. Words with a long stem such as "radio-" are often stressed on the syllable before the suffix, and "-ive" itself is almost never stressed. Words ending in "-ive" often take stress on the syllable that begins the "-active" part, as in ACtive, reACtive, interACtive.

Step-by-step solution:

1. Break the word into syllables: ra-di-o-ac-tive (5 syllables).
2. The word is built from radio + active. The base word "active" is stressed on its first syllable: AC-tive.
3. When radio- is attached to active, the stress falls on the "ac" syllable. This follows the same pattern as interACtive and reACtive.
4. Check the other options: • A. RAdioactive: stress on the first syllable, which is not the natural pattern. • B. raDIOactive: this is the pattern many dictionaries give (/ˌreɪdiəʊˈæktɪv/), with primary stress on "ac". The "di" syllable carries at most a secondary stress, so B is not correct. • D. radioacTIVE: "-ive" is a weak suffix and is not stressed.

Answer: C. radioACtive

The primary stress is on "ac", and the first syllable "ra" takes only a secondary stress (/ˌreɪdiəʊˈæktɪv/).

Tip: For words ending in -ive, never stress the suffix. Find the base word (active) and keep its stress: acTIVE-type words follow AC-tive, reACT-ive, creATive. Remember also that "-ive" words never get stress on "-ive" itself.