Triple fusion in Capsella bursa pastoris is fusion of male gamete with.

The correct option is C Secondary nucleus
The pollen tube penetrates the embryo sac and enters the egg apparatus. Ultimately, the tip of the pollen tube bursts and both the gametes get discharged. One of these gametes fuses with the egg cell of the egg apparatus. The other gamete fuses with the secondary fusion nucleus. This behavior of the two male gametes is termed double fertilization. The secondary fusion nucleus of the embryo sac, how­ever, normally becomes 2n at the time of the fusion undergone by it. So, the fusion of this nucleus with the second male gamete is triple fusion and the resultant nucleus is triploid or 3n.

So, the correct option is 'Secondary nucleus'.