Convert categorical variable into binary columns in R

I made the stupid mistake of enabling people to select multiple categories in a survey question.

Now the data column for this question looks something along the lines of this.

using the following line in r,

datanum <- data %>% mutate(dummy=1) %>%
spread(key=answer_openq,value=dummy, fill=0)

I get the following:

However, I want the dataset to transform into this:

Any help is appreciated (my thesis depends on it). Thanks :)

Try this:

library(dplyr)
library(tidyr)
df %>%
  separate_rows(answer_openq, sep = ',') %>%
  pivot_wider(names_from = answer_openq, values_from = answer_openq, 
              values_fn = function(x) 1, values_fill = 0)
# A tibble: 4 × 5
  respondent     a     c     b     d
       <int> <dbl> <dbl> <dbl> <dbl>
1          1     1     0     0     0
2          2     1     1     0     0
3          3     0     0     1     0
4          4     1     0     0     1