To determine which set of elements delays flowering when their concentration in plants is low, we need to analyze the role of each element listed in the options.
### Step-by-Step Solution:
1. **Understanding the Role of Each Element**:
- **Nitrogen (N)**: Essential for plant growth, its deficiency can lead to chlorosis (yellowing of leaves) and dormancy of lateral buds. It is important for overall plant health.
- **Sulfur (S)**: Important for protein synthesis and chlorophyll formation. Its deficiency can also lead to chlorosis and delay flowering.
- **Molybdenum (Mo)**: Involved in nitrogen fixation and enzyme function. Its deficiency can inhibit flower formation and lead to chlorosis.
- **Potassium (K)**: While important for plant health, it is not directly linked to delaying flowering.
- **Calcium (Ca)**: Important for cell wall structure and growth, but not specifically linked to flowering delay.
- **Magnesium (Mg)**: Important for chlorophyll production, but not directly involved in delaying flowering.
- **Copper (Cu)**: Plays a role in photosynthesis and respiration, but not directly linked to flowering delay.
2. **Evaluating Each Option**:
- **Option (a) N, K, Mg, S**: Contains nitrogen and sulfur, which can delay flowering, but potassium and magnesium do not contribute to this effect.
- **Option (b) N, S, P**: Contains nitrogen and sulfur, but phosphorus (P) does not directly delay flowering.
- **Option (c) Ca, Mg, Cu**: None of these elements are linked to delaying flowering.
- **Option (d) N, S, Mo**: Contains nitrogen, sulfur, and molybdenum, all of which are known to delay flowering when in low concentration.
3. **Conclusion**:
After evaluating the options, the correct answer is **(d) N, S, Mo**. This set of elements is known to delay flowering when their concentrations are low in plants.
