(d) Watermelon and Papaya
Papaya and watermelon bear unisexual flowers (containing either stamens or pistil, not both), while mustard and Hibiscus bear bisexual flowers.
Source: Chapter 7, Section 7.3.2 — Sexual Reproduction in Flowering Plants
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The textbook explicitly states: "The flower may be unisexual (papaya, watermelon)… or bisexual (Hibiscus, mustard)." So any option containing mustard or Hibiscus refers to bisexual flowers. Only option (d) lists both unisexual examples. Remember: unisexual = one reproductive organ; bisexual = both stamens and pistil.