Which ending correctly completes the unseen accusative sentence: اِشْتَرَيْتُ قَلَمًا ____?

Causes of Ghayr Munsarif: Wasf and the Pattern Af'al

• أَحْمَرُ
• أَحْمَرٍ
• أَحْمَرٌ
• أَحْمَرَ