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Explanation:

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Let AE be a perpendicular from A to DC.

∆ADE is a 30-60-90 triangle.
⇒ DE = √3/2 × 10 = 5√3
⇒ AE = 10/2 = 5

∆ACE is a right triangle.
⇒ CE2 = AC2 – AE2
⇒ CE2 = 400 – 25 = 375
⇒ CE = 5√15

∴ Area of parallelogram = 2 × Area of triangle ADC
⇒ Area of parallelogram = 2 × (1/2 × CD × AE) = CD × AE
= (DE + EC) × AE
= (5√3 + 5√15) × 5
= 25(√3 + √15)

Hence, option (c).

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