In a monohybrid cross what will be percentage of heterozygous offspring in $F_4$-generation-

The correct option is D 6.25%.
Suppose we take the genotypes Rr, in F2 generation we will get 2Rr, RR, rr, now in F3 generation a cross will take place between RR X Rr, Rr X Rr, Rr X rr, RR X rr, progeny will be 3RR, 3rr & 10Rr. For f4 generation percentage will be 10/16x 100= 6.25%.

So, the correct option is '6.25%'.