如果Type始终是seqA， B：

df <- data.frame(data.table::fread("Year Type Return
1900 A 4
1900 B 7
1901 A 87
1901 B 3
1902 A 9.7
1902 B 2"))

library(tidyverse)
df %>% 
  group_by(Year) %>% 
  summarise(Return = Return / sum(Return)) %>% 
  ungroup() %>% 
  mutate(Type = df$Type) %>% 
  relocate(Year, Type, Return) %>% 
  mutate(Return = round(Return, 2))

   Year Type  Return
  <int> <chr>  <dbl>
1  1900 A       0.36
2  1900 B       0.64
3  1901 A       0.97
4  1901 B       0.03
5  1902 A       0.83
6  1902 B       0.17

df %>% 
  group_by(Year) %>% 
  summarise(Return = Return / sum(Return)) %>% 
  ungroup() %>% 
  mutate(Type = df$Type) %>% 
  relocate(Year, Type, Return) %>% 
  mutate(Return = scales::percent(Return))

   Year Type  Return
  <int> <chr> <chr> 
1  1900 A     36%   
2  1900 B     64%   
3  1901 A     97%   
4  1901 B     3%    
5  1902 A     83%   
6  1902 B     17%