Which bond would you recommend for an investor whose time horizon is $8$ years and who has moderate risk tolerance?

a$.$

AAA rated, $5\%$ coupon, $7-$year maturity, duration $5.91$ years

b$.$

B$+$ rated, $5\%$ coupon, $12-$year maturity, duration $8\mathrm{.}23$ years

c$.$

AA rated, $4\%$ coupon, $10-$year maturity, duration $9\mathrm{.}68$ years

d$.$

AAA rated, $5\mathrm{.}5\%$ coupon, $15-$year maturity, duration $10\mathrm{.}43$ years