Assertion (A): In angiosperms, endosperm is always triploid.
Reason (R): Endosperm results from syngamy.

The correct option is C A is true, R is false
In angiosperms, out of the two male gametes produced by each pollen grain, one male gamete fuses with the female gamete to form the diploid zygote. This process is called fertilisation or syngamy.

The other male gamete fuses with the two haploid polar nuclei. This results in the formation of triploid primary endosperm nucleus (PEN). The fusion of male gamete with polar nuclei is called triple fusion. The cell that consists of PEN is called primary endosperm cell (PEC). The PEC undergoes further division to give rise to the endosperm.
Thus endosperm is always triploid and it is formed as a result of triple fusion and not syngamy.